I did some tests and I think I know how the whole crew thing works in Single Player.
First, most crew members can possibly appear in 2 levels. Their appearence is totally random and it's not related to the mission level. Either a crew member appears or nobody appears. There are actually 3 crew members from single player that increase health: Brute A, B and C.
Brute A can be found in level 1 (area where the last ring is) or 2 (starting area). It's easier to find him in level 2. The fastest way to find him is to load level 2 and immediately activate the sonar. If a purple dot appears, go get the guy. It might be Brute A or not. If no purple dot appears, press start and restar the level. Do this until purple dot appears. After you get the crew member, you need to complete the mission.
Brute B can be found in mission 3 (radar base area) or 5 (starting area). Again, its easier to do in mission 5. Repeat the same process you did for Brute A.
Brute C can be found only in mission 7. Do the same process you did for the other Brutes.
Remember, it's totally random which crew member will appear and if it will appear at all. Be prepared to farm a lot
.