Before posting make sure you are aquainted with our forum rules.
Mooh guest join to room and leaves in frist round. This game need no guest option
Anybody can leave the room at any time and even at teamgames, where you punish the whole team and destroy your rank it happens quite often. There is no other case for guests. As guest you can just join non-premium rooms, so they just can play at unranked rooms with max. 6 players. This is the case for normal FFA games, where you would have one opponents less or 1vs1, where you get a free win. They also can join unranked teamrooms, but just for the case that they are added to an account, so they both would have to leave.
If you don't want to play with guests you can simply kick them as host or as normal player search rooms without guests.