How do I control who enters a game in multiplayer

I'm creating a multiplayer game and want players to be able to pick the character they want to play. One person is overpowered and the enemy, the other players are weaker and work together to defeat the overpowered player. How can I make it so that there is only one person who's overpowered like that? Or if this is simpler, how do I restrict what players can enter a room based on the other players already in the room?

The simplest way would be to make the unique player the host. If you join a game, you will be part of the team, if you host the game, you will be solo.

Well if a person wants to do that how could I make it so that a person can create a game and be the host instead of just joining a game?