when u join a game and u choose a team and someone else joins who is not ur friends joins ur team when the game starts the person in ur team gets replaced with ur friend? i have had this happen lots of time
^^^Decide on a team and quickly join and get into it. If your friend didn't join it in time, you can /leave and try again.
That is what I do with my friends. I don't think it's nice to replace someone just because you don't want them there.