In SPACE... the lowest numbered actor will appear in a single player environment. The other actors for that group will appear if you have teamates and will move down the list according the number of teamates. Difficulty settings have no effect on this.
On the GROUND it is a different story - all of the actors appear all the time because you either A) have your BOffs or B) you have teamates or a combination of both equaling 5 members on a ground map all the time. Again - difficulty settings have no effect on the number of actors.