Mmm, generally you join what you want of your own free will, if you have a look on the regiment sub forum (http://forums.taleworlds.com/index.php/board,283.0.html) you can see all of the regiments.
If I were being biased, I'm with the Freikorp (http://forums.taleworlds.com/index.php/topic,223707.0.html) and we happen to keep an artillery detachment who join line battles regularly, but there are others out there as well, the 77y keeps a nice one if my memory serves.