From my experience with the private server option unless it was fixed, you can practically join a public game with a whole team. My friends and I kept trying to play some 5v5 games, but it kept tossing our entire game into a public match. We would join a random mid session game and be like wtf who are all these people. It kept the same format of our teams we set though. So, I never tried, but maybe you can put everyone on the same team and start it up.