Is there a command that I can macro to target the mob the main assist is targetting? Or do I need to type his name in for every group? Or is there another way?
/assist group /assist raid The former will assist the group's MA, while the latter will assist the raid's MA. (If there is one, of course. The people in question have to be formally flagged as the Main Assist.)
Setting your "raid assist 1-3 target" or "group assist's target" in your extended target window also allows you to use the /xtarget (ex: /xtarget 2) commands in macros.