[RPG] Scaling a game for a large group

Check out this related question Is a high level Elite the same difficulty as a low-level Solo, as XP suggests?

use the xp encounter guidelines laid down in DMG2 to craft an appropriate encounter and only use enemies within 1 lvl of the party (4-6 in your case)

The key reason enemies should only be within a lvl of the party is that 4e's very planned progression/math expects certain numbers for to-hit, defenses, health and damage for both the PCs and monsters. If either side is too high compared to the other they will avoid being hit and land lots of hits dealing a lot of damage to the smaller health pool of the lower level side.

As you can see for 5 lvl 5 PCs an encounter budget of 1000xp is considered standard. A lvl 5 solo would perfectly match that xp budget.

Once you've found the standard XP for your party size and lvl then look at the difficulty table to increase or decrease the xp budget accordingly

Scale it up, make the budget 1250 or 1500 and use a lvl 5 solo mixed in some other enemies to really put on the pressure.

Use the standard budget, but pick enemies that prey on the party's weaknesses

The party might be too ranged-centric, rely on too many power combos or have a specific NAD that is quite low. You can make things more difficult by creating encounters specifically to undermine their strengths and exploit their weaknesses.

It sounds like you have two problems: (1) your game with 6-8 players has slow combats unless you use a small number of monsters, and (2) if you use a small number of monsters, your crowd-controllers can shut the encounter down.

The first problem is easier to fix:

• Use a larger number of monsters, but make most of them them identical so that you can run turns for them quickly. Move them all on the same initiative. "Okay, these three orcs are attacking you. Each of them makes one attack, with an attack bonus of +4... (rolls) they hit AC 11, 15, and 17. How many hits? Okay, take... (rolls) 12 damage." Optionally include one or two bigger monsters so that your crowd-controllers have something awesome to do.
• Consider splitting your game into two smaller games. You can start slowly -- have occasional "side missions" which don't involve the whole group. These will be easier to schedule: choose the time and place that work best for you, and see if around half of your players are able to be there. If that seems to work, you can have more and more side missions, and save the "full group" scenarios for boss fights and special occasions. In the extreme case, this turns into a West Marches scenario.

For the second problem: remember that D&D 5e is balanced around having a lot of encounters per day. Are you doing that? If you're only giving them one big fight in between rests, your characters with daily powers will be much more powerful than the designers intended, because they can use all their dailies in that encounter. Those crowd-control abilities: are they daily powers?

If you suddenly change your game from one fight per day to three or four, make sure to telegraph that in advance so your casters don't waste all their spells on the first fight.

One final note: it can be dangerous to have battles with a small number of active monsters. What tends to happen is each monster stands in one place and focuses all its damage on one player character. This is really bad for that player character, and it's sort of boring for all the other characters who never get attacked. One solution I've used for this problem is, when I have an encounter with just one monster, I make sure that all its attacks are area-effect attacks, so that it spreads out the damage more evenly. This involves a lot of inventing homebrew monsters, though, so it may not be best for every group.