Hi Boone,
In bp_group_management_add_member_list() you output a <ul> tag on line 427, followed by some nested div tags (line 468-470), then another opening <ul> tag to start the list of addable members, then output the two closing </ul> tags on lines 492 and 494.
This causes invalid markup, and the layout of the add members list is broken as a result.
Kind regards
Chris