NEED help with adding members to different lists

I am VERY new to wordpress. I do not understand shortcodes completely but i managed to edit shortcode a bit on the pages i posted to get rid of useless stuff for me like zip code.

i have two member lists A and B. I want to make it so when members register or even after they already registered can choose to be in list A or B. for some reason i cant figure out how to put members in a list. all my members are in list A and i want some to be in list B also. i couldnt find an option for picking who goes anywhere in plugins menues or anywhere!

how can i do this? im new to this so explain clearly please.

http://wordpress.org/extend/plugins/members-list/