Crew points
To unlock some of the higher level missions, you must have crew points.
There are several different ways to get Crew Points.
The best way is to invite your Facebook friends to join the game. For each NEW player who joins the game through your invite, you gain one "super-droid" with their name on it, which provides a 4% bonus to the building it is assigned to, can be assigned to any building, and also provides 3 Crew Points.
Another method involves spending credits to buy an "android". These are a generically named "super-droid" with abilities identical to human Facebook friends.
The final, and most common method, involves building a Factory of the proper level, and then building Crew Droids. Each Crew Droid provides 1 Crew Point, but no other benefits.