Create a crewman and a sniper. Set crewman to 'player', rank 'corporal. Set sniper to 'playable', rank 'private'
Create an empty Hummingbird.
Test the level.
The crewman will be able to sit on any position, but the sniper will only be able to take the co-pilot seat (once the pilot seat is filled).
Now go back to the editor. Swap the ranks (so crewman is now private, sniper is now corporal).
The problem is now reversed - sniper can sit anywhere, crewman can only be copilot.