If it's your first time GMing I truly suggest you begin with a smaller group. Gamesmastering is a skill. Handling a group of players (all talking at once, being distracted by other things, etc) is not trivial.
At least make the group appoint a spokesperson. One player that is the one you talk to, and have the other players talk to him or her. You're seven people around a table - you shouldn't attempt to listen to all of them like you can with 3 or 4 players.
Also delegate tasks. Tracking initiative and monster damage are two examples of tasks you can and should have one or two players handle (writing down, calculating, erasing, changing etc).
Consider running a "theatre of the mind" game, meaning don't bother with a battle map or keeping track of exactly in which square each hero is. Yes, this means some abilities (such as class feats) lose a bit in power, but as long as your players know this beforehand they know what they're getting into.
Don't worry about game balance and game challenge for your first game. Just run the encounters as written. Sure, monsterkilling becomes easier with 6 heroes instead of 4, but you can always think about making combats harder after your first few game sessions. It is not the thing you should worry about first!
Good luck!
