Scaling for a large group

I also recommend giving every enemy NPC max hit points. Large groups of players can focus fire and bring down enemies quickly.

I find with 8 or more players to both increase the number of enemies and max out hit points.
More enemies is usually better than increasing CR but YMMV. I make sure enemies have ranged weapons even if the stat block normally doesn't normally have one.
Throw in a few extra oddities as well. The monster a day set of monsters had some interesting enemy monster variants that help keep battles interesting.
Give enemies tangle bags, alchemist fire, or bottles of acid to throw.

Add interesting terrain that forces players to get creative.

practicalm said:

I make sure enemies have ranged weapons even if the stat block normally doesn't normally have one.

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Yes, at least give melee creatures some javelins for when they can't reach the foe. Or a troll might have proficiency in throwing rocks - 2d4 damage but a +4 STR bonus still makes it annoying.

DEFCON 1 said:

One thing that I think often goes unmentioned in situations of large parties like this (and is something I've found to be pretty standard and typical) is that the more PCs you have... the most people available to get PCs stabilized and/or back on their feet.

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This is definitely true, and a function of bonus action & 4e minor action healing. You get a kind of quadratic effect. Monsters can counter it by all going at once and finishing off fallen PCs immediately they go down, but I tend to reserve that for after the monsters first see the PCs bringing fallen PCs back up again.

1. Ban spellcasters.

2. Everyone get to have "heal" spell. heals 5 HP per level. Number of times per day equal to your proficiency bonus.
Action to cast on someone up to 30ft. Bonus action to cast on self.

I had 8 players for a few years in 5e. Even though the WotC adventures are for roughly 5 heroes, I always calculate them at 4 for the purposes of scaling the encounter when I have a different number of heroes than 4. Since you have 8 heroes, I would suggest doubling the number of enemies in each encounter. Named/unique creatures get 20 hp for each additional PC instead of being doubled (30 hp when they are alone). For the dragon, since it is on its own, add 30 hp for each additional hero in your group.

The benefit of this is that the difficulty of encounters will be roughly the same. The downside is that encounters will be loooonger, especially with 8 players. Make sure everyone knows what they are doing when their turn comes up. Even at 1 minute per player, it will be 8 minutes per round just to get through them (and they almost never that fast).

Do things to save time:
* Have them roll damage dice with their attack rolls.
* Have another player take care of tallying initiative and put it on a small white board or something else that everyone can see so they know when they are coming up and no one gets skipped.
* Have an idead of what you are having the NPCs do ahead of time.
* Consider using average damage rather than rolling it.

With that many players, stronger monsters won’t help thst much. Your players will still mulch them. My group of 6 handles CRs +5 all the time without even breaking a sweat.

More monsters is really the way to go, or focus on area attack monsters. Also, active traps can be great here. A trap can be set up to hit multiple parties, and can be a good change of pace compared to “every enemy has a fireball slinger”.