That premise sounds really similar to a videogame I played a while ago, but I forgot the name. 

Anyway, I don't have a lot of experience with NRPs, but if you're planning on doing the RP with friends, just be easygoing. Shoot them a PM if they stop showing up, be firm (but polite) in your expectations. If any one of them tries to derail the RP with ideas of their own, go with the flow a little bit, but don't let them sidetrack you completely. 

Write your posts in past-tense, omniscient third person ("Mark's colony was doing well, until they were confronted with the sight of raiders appearing at the horizon. The raiders were..." etc etc). That's probably the easiest way of getting information across without breaking immersion.