Press ` to get the console up.
cl_selectm (marinenum) (slot) (Brackets arent needed by the way)
(marinenum) being numbers 0-7 for Sarge, Wildcat, etc. through Vegas, respectively.
(slot) being 0-3, for players 1-4, respectively. Slot number -1 can also be used to add to the first available slot.
cl_dselectm (marinenum) (slot)
to remove ones that arent what you want.
Sarge "cl_selectm 0 x"
Wildcat "cl_selectm 1 x"
Faith "cl_selectm 2 x"
Crash "cl_selectm 3 x"
Jaeger "cl_selectm 4 x"
Wolfe "cl_selectm 5 x"
Bastille "cl_selectm 6 x"
Vegas "cl_selectm 7 x"
x being the slot you want them in - ideally 3 or 4, that way you and the other play are 1 and 2.
========
F2, f3 or F4 to control them ingame if it appears needed.