I don't know how this could possibly be put into the game, but maybe there could be like a friends option in the gui menu when you start the game, then when you want to add someone as friend you could type /add friend username or right click their name or something, of course the other person would recieve it as a friend request and then they could accept or deny it. I think it would be really cool to have this option, because then you could have like and invite option or something. If any of you want to add onto this feel free.