Allow late-arriving player to join game
If a group knows they'll have a set number of people but they're still waiting on one or two, you should be able to let that person join after the game starts. For instance, if you have 3 but know you'll have 4 you start quads. When that person arrives, they can join the team. If the gulag is still open, they either enter the gulag or just parachute in.
Will the missing players be an AI when the game starts? What if these players decline the game? The game would also have to be a practice game if they start as an AI.