You simply make an "auto-booter" if the player is occupied by AI. So in the lobby you fill up all the remaining slots with AI, and when the game starts, the non-human players that are supposed to be human will get kicked. This is easily done by having for example a player flag near a gaia object. If the player is human, the gaia object will be converted to the player, while an AI will not convert the gaia object. So simply check object ownership a couple of seconds in to the game