The biggest problem I've had with groups of 8 is not balance but that players minds tend to wander when the focus isn't on them. This means that they generally aren't ready for thier action and weren't paying attention when their turn in the round occurs, which means extra explaining, questioning, dithering, and delay which means that the other players minds wander even more.
Keep eight people entertained simultaneously is tough. I think 4-6 is ideal, and I thought so even back in 1st edition.
Another big problem with eight players is social dynamics. Nine people is a fairly large group, and typically they split into two circles of closer friends. Character and sometimes player rivalry can be a big issue, even with good friends. Even if it just stays IC, party cohesion can be a serious problem if you've got more than one 'Chaotic Greedy' character/player, and more than one person with a e dominate personality. Expect alot of debates about what to do. I strongly suggest you force these debates to be IC.
But as for balance...
Eight players will wipe out any single low ac creature very very rapidly, regardless of how many hit points it has. Equally, anything big enough to challenge eight characters simultaneously by itself will smash any character it gets its claws on.
The best bet is to either double the number of foes you'd usually offer, or else add a couple of minions or companions to divide the players attention. If previously you'd challenge the players with an CR 8, add four CR 4 minions to divide the party up. That way you are less likely to overwhelm any one player but you give them all something to do.
Word of advise though. The CR system is not at all accurate. In particular when it comes to increasing the number of monsters, IME doubling the number of creatures with a CR more than one lower than the players level doesn't really double the challenge or increase the EL by two. It's really more like increasing the EL by one. Large numbers of mooks are generally speaking just a large number of mooks, and present no really harder challenge whether there are 16 or 32 of them. Depending on how skilled your players are, six CR 4 minions plus a CR 8 might be more like twice the challenge.
Now, there is one more issue and I almost hestitate to bring it up as I think its something of a dirty little secret of DMing. As a DM, you can't always play the monsters as 'smart' as you are capable of. For one thing, if the monster is low intelligence, its not even appropriate. But even if the monster is reasonably intelligent, there are some things that the DM should avoid particularly with large groups. I reasonably skilled large group will concentrate thier attacks on a single opponent, preferably one that represents the greatest threat, and of those preferably the one with the lowest hitpoints or lowest armor class. The goal will be to deny the opponents actions by killing them. By all means resist the temptation to do that to PC's with the monsters unless the players are just getting too cocky. While it is or ought to be the monsters goal to kill the PC's, it certainly shouldn't be your goal. The PC's will do plenty to get themselves killed if you are doing your job right without certain sorts of ruthlessness on your part. This is one of them. Not only will you certainly kill characters if you just set out to do it, but in my experience some players will take monsters ganging up on them and thier character being singled out personally. It's just not worth it. Distribute the foes about so that everyone gets banged on somewhat equally unless someone is sticking out like such a sore thumb that not ganging up on him will give the fact that you are metagaming away.
Celebrim's rule of DMing #159: PC's should only die when thier player feels that in some way they earned it.
In a since though, not perfectly coordinating your monsters isn't metagaming. In the real world, its almost impossible to perfectly coordinate any large group. In reality, there isn't a puppetteer pulling all the strings with perfect knowledge of the situation. So not perfectly coordinating your attacks is really part of playing the monsters well. But even if it wasn't, I'd still recommend against it except when the flavor of the monster really warrants it (Modrons, axiomatic creatures, high intelligence lawful outsiders in general).
Which is another way that the CR system is just wrong. Played well, or what I think constitutes playing a creature well, a creatures intelligence is very much part of its CR. Zombies, oozes, plants, animals and the like probably should get lower CR simply because played well, they aren't going to be tactical masterminds. Equally, while the players encounter a high intelligence being, they should notice how much more cunning and skillfully it conducts itself.
