Why not both? A coop mode AND a server mode. One for smaller groups who want to start at the same time, and one for people To join and 'jump ship' at will, but with a different entry mode to make it more compatible with such a large number.
Well considering that one of the goals is to seal the breaches in the dark matter containment system, which only needs a lead lined suit...perhaps instead it'd be more along the lines of finding enough left over materials to make some self charging …