Who's the leader in the group?
I don't think it's Alvin or Rebecca or Sarah, but I don't think it's openly stated who the leader is.
It could be Luke, as Clem does say something like "you're the leader here right?" if you try to beg Luke or something like that.
It could be Carlos, as it seems he has a big hand in the decisions.
It could be Nick, as he's the one with the gun mostly, but he seems like a dumbass.
Could be Pete, as Luke calls him "Sir."
I originally thought it was Carlos, but then I heard the dialogue you can say to Luke, and now I'm just confused :P