Yeah, well sometimes I have the chance to board a helicopter (when a teammate is kind enough to wait and doesnt just fly away even if he can see me running towards him :) )
But mostly teammates just hang around the parachute map and *swoosh* - They disappear into nowhere. I figured out that in most cases they parachute over the next target area on the missions list - So this will help for now (aside of just asking where they are - which actually doesn't work often anyway :( )