First of all, all I have is the Beginner Box.
My group was small at first. 3 Players, a friend and two of my brothers. Since then, four other players have joined. Most of the campaigns I had planned are for 4 players maximum but I don't want to deny anyone. I made extra blank beginner character sheets for the new players and now it seems the group may become an 8th because of one of the players' girlfriend.
What I'm wondering is how do I adjust the campaigns accordingly and I could use any general advice for GMing games over four players.
Holy crap man eight is enough! At that point you have to plan on double everything, including the amount of time to finish an adventure, say if you normally have an adventure last 2-3 sessions now it will be 5-6 sessions of the same length. With that many people you will need to delegate small jobs for these folks to help you out, like have someone keep track of initiative someone else track party conditions etc.
My advice would be to consider splitting into 2 groups. Could someone else run one of the groups so you're not having to run 2 campaigns at the same time? That might be ideal (and give someone else a chance to run).
If not, you could just run the same material (recommend an adventure path for sanity's sake) for both campaigns in some sort of parallel universe if you will. That way you're not having to duplicate your efforts quite as much.
Running for a group of 8 is not recommended unless you're very experienced. Combats take forever and players get bored in between turns. It's hard for individuals to shine due to all of the players around the table.
Some of your players may just be testing the waters and decide not to play after a session or two (no fault of your own, it may not be their cup of tea) so bear that in mind (it's another reason I wouldn't plan on playing with 8 players and designing adventures accordingly, for example). Good luck!
Only two of the players are actually willing to do a group split, the other six (my two brothers, three friends and then the girlfriend) all want to run in the same group. The other two, two very close friend of mine, are brothers and one of them can't leave the house for long (medical conditions) so I'm considering doing a 2 player campaign, maybe three if their other brother decides to join, at their house on wednesdays. The other group is only free on sundays.