Yes, bombardier and pilot just click on these places on pilot Blenheim.
Sample - online on ATAG now:
Just click on vacant (user) places on Fair Light Blenheim.
Is need that these three player select the same airfield.
In rares occasion you are able to join as gunner in AI bombers.
BTW - Notice that 92 Cdt.Coldstreamer player occupies Pilot and Bombardier places in his Blenheim.