I routinely run with 7+, often as many as 10 or 11, and have for over 20 years. There are differences in running this way, but the main reason it doesn't work for many people is that they don't have much practice doing it. People try it once or twice, doing everything the same way they did with 4-6, and of course it doesn't work.
So first thing, ignore any advice from people telling you that it
cannot work. They don't know what they are talking about. Instead, take any such advice as, "it didn't work for me, and you might not be willing to do what it takes to work for you," which has the considerable benefits of being both true and more useful advice.
Everyone has a number of players with which they are most comfortable, and a DMing style that they are willing to bend or not bend to stay comfortable. Find where you are. (As it happens, I've run so long with a big group, I'm not really comfortable running with less than five any longer. I can do it, but I don't enjoy it very much.)
With that said, here is my concrete advice for making it work:
1. Most important, the players
must help you make this work. If they won't, you'll fail. If they don't know how, they must be willing to learn. If individuals aren't willing to learn how, then you'll have to either drop them or split them into a smaller group. Nothing that comes later will matter if you have half the group insisting on playing like there are 4 players at the table.
2. "Attention Fatigue" is your enemy, for you and the players. Even two hours of juggling 8-10 helpful players, will leave you drained, if you don't have some kind of break. Meanwhile, the players all want your attention, and are tired of paying attention to someone else while they are out of the action. But you don't want the game to stop, and you'd like the "breaks" to flow naturally. One of the best ways for you to get a break without stopping the game is to get the players to roleplay with each other.
Consider the usual, "split up in town and research the rumors at the taverns, town hall, library, etc." tactic. You can ban the split. Or you can have 10 people run off in 10 different locations, and drive yourself crazy. (And them with you, because that is splitting the "fun" 10 ways into tiny slices.) Or you can just flat tell the players (OOC) that to make this work, you'll have 2-3 scenes of gathering information roleplayed, and you want a fairly even distribution of characters per scene.
Then, you've only split the fun 2 to 3 ways. But you don't stop there. The mage and the cleric decide to go to the library. The ranger, having nothing better to do, helpfully decides to round out that group, and keep watch as the "strong, silent" type. At the library are two NPCs. If you saw it coming, you have a small handout that says what they know. You give this to two other players, and tell them to roleplay the scene. (If you need to wing it, you pull them aside and give them the information, quickly.) Bingo, you've got half the group involved in the "personal" stuff.
Now, at first you probably want the rest of the group watching. Don't worry that they get information they wouldn't have yet. You can't be a stickler about that kind of thing. Besides, for everything they learn OOC this way, you'll have another thing that confuses them simply because of the communicating issues with a group this size. It evens out. So if one of your other players handling an NPC screws up the information delivery, but knows the answer, and this leads to a misunderstanding ... that's a feature, not a bug.
By the same token, don't pay that much attention to what is going on. This is your chance to rest for a bit, or mentally get ready for the next scene (or more likely, a combination). When you really get cooking with this style, you don't even wait. You just take the other five players and go another scene with them while the first sub group does their scene without you. And don't go into another room if you do this. The goal isn't to hide information but to expedite play. If your second scene at the other end of the table gets derailed for a few minutes because the first scene gets really interesting to the whole table--hooray, the whole table is entertained.
Note that none of this says, "drag it out." When you have 4 players, the other three don't want to sit for an hour while the mage talks solo at the library. Why would half the group want to sit for an hour while the other half does the same thing?
3. Prepare encounters as if you had two separate, more usual sized groups. Do this explicity. This is an extension of the, "don't use a single, tough solo" advice mentioned by others. You should also do this with skill challenges, and in fact most elements (e.g. treasure). Not only does this make it easier on you to prepare using the standard rules, it covers you if a bunch of players don't make it one session. You can just run with half of the prepared material, saving the rest for later.
Note that you can get really clever with any system to make this easy (instead of double prep), and 4E is especially nice in this regard, with working encounter budgets and encouragement for reskinning.
For example, let's say that I have 3 kobold encounters planned, and might need 2-3 more, depending up what the players do. Do you need 12 encounters planned, #1 and #2 together, then #3 and #4 together, etc? No, you need about 4-5 different standard ones! Because combinations are more varied than individual pieces. Your set encounters are maybe standard pieces #1 + #2, #1 + #3, #2 + #4. Plenty of variety. If the players surprise you, and you need another, just grab #1 + #4, or whatever appeals, reskin if desired, and go. And note that it is perfectly fine if one of your "different" encounters is a solo (or mostly solo). You are not going to use it by itself, unless half the group doesn't show. This also provides you an easy way to really rachet things up with waves of monsters. You set things up so that if the players are exceptionally clever, they get to steamroll your encounter A (#1 +#2) in separate pieces, if they are average, they get it as written, or if they screw up, they get a #4 pieces added in a few rounds later.
4. If you've got players that are willing and able (or want to be), let them play some of the monsters in big combats. You keep the monster leaders, but just hand out the stats for the rest. Some players prefer not to handle the monsters that are directly confronting them, but you can easily get around this by dividing the characters into two squads that tend to act together (or three if this is more natural).
5. Push the players tactically, even more than you would in a normal game (with whatever gradual push or learning curve is appropriate for any beginners in the group, of course.) Especially do this if the players are happy helping with monsters. Because one of 8 characters being temporarily taken out of the action, or even killed, is not the threat to TPK that losing one of 5 characters is. As long as the player can be entertained by something going on in the game, you are good. OTOH, if you do this, make sure the player has a way to get back into the swing easily. (That is, don't make him bring in a new character 2 levels lower, don't make him wait to bring it in, etc. If all the hokey, "we rescue a prisoner that just happens to be your character" story lines bother you too much, you'll have trouble with large groups. You don't have to dwell on it, but you do have to get a viable character into that player's hands ASAP.) Also note that this does not mean that you should use full-blown metagaming tactics from the monsters. Don't have
all the monsters focus fire on the mage. You've got at least two normal encounters' worth of monsters on the table.
There are two other, related reasons why you want this extreme push. First chaos multiplies in large groups. Chaos is your friend, because it is the main thing that keeps it exciting for the players. Without chaos, you'll soon find that 8 PCs running like a well-oiled machine versus two normal budget groups or monsters is often a cake walk. All those powers that magnify the power of the group (e.g. I'm looking at you, Tac Warlords) are just the tip of the iceberg. So ideally, you'd like your players to handle their characters with a little friction. If two PCs have a rivalry, and the players can be mature about it, it's ok. The group can survive this.
Second, any rivalry and friction is going to naturally feed back into the players roleplaying with each other. If the warlock and the rogue get into a shouting match (in character) in the middle of the fight, that's good!
6. Reward players that help you do this. Be explicit with the rewards. I don't much like bonus XP, because I like the group to more or less grow as a group, but any reward that works for you is ok. In 4E, I hand out bonus action points for any action that significantly brings in other players into the scene. It could be as simple as the mage turning to the paladin, during a scene with an NPC, and asking (in character), "What you do think?" Or, "What does the Order of the Silver Blade say about that?" Immediate action point for the mage.
Don't worry if not everyone gets these rewards, either. Out of that many people, some will be wall flowers. But you aren't short changing them. On the contrary, you are helping them. You are explicitly saying that players who drag them out on the dance floor get more powers to ... keep doing this.
I hope this helps.
