Fleets.
That said said, I think crews should be pseudo-permanent and fulfill the roles traditionally taken by the "guild." But once players get large enough, or players favor having their own ships, then the 'crew' becomes too large to be on one ship and becomes a 'fleet.' Though, in the interest of not having the foolishness of having 5 fleets on one ship, I suggest using the term 'subfleet' and using fleets to designate large sets of ships. Even if the fleets are customizable, it'd be nice for my own immersion if a 'fleet' had a minimum number of ships and people to be called a fleet, even if players can form "subfleets," "fraternities" or "Squads" at any time. Squads sounds kind of fun...
