Difficult one that is to answer..
Probally military rank is the most recognisable, Nasa 'ranks' would work and be more in line with the sim (Mission commander, 'flight/mission surgeon' , co-commander etc)..
How about if the option was given to mission commanders to either enable roles or allow everyone to do as they wish (would allow for serious planned missions, or 'open server')..
It may be the case that players would want to join and do there own thing, or players would like to start as rookie and build up there experiance to do other specific jobs..
It could be problematic to have 10 rookies running about, being frustrated cause they cant (not allowed) to use a rover etc, when the mission commander is offline, also in the same context you wouldnt want rookies running amok with rovers etc..
I say mission-commander can either allocate jobs(roles) to people they wish or players can join and 'rank up' (say by taking tasks/missions from hab PC ie 'grow 6 plants or collect 10kg ore etc') to allow them selves freedom of useage of gear etc..