Vyndree has a really good video showing how to do this. (Go to about 6:00 in the video.) Basically, you bind different movement keys on different toons to one key.

If you want a formation like this:

------B-----
C----A----D
------E-----

where A is your main, then you'd have to bind B's move forward, C's strafe left, D's strafe right and E's move back to the same key. Reverse the directions and use a different keybind to close the formation.