Basically, check your register.php main loop and only show the groups you want to show.
You’re looking for the while loop after the bp_is_active( ‘xprofile’ ) call.
Within the loop, you can filter out groups by doing a simple if check using
if (bp_the_profile_group() == "group number") where group number is the number of the group you want to show.
Similar sort of idea in this thread: https://buddypress.org/forums/topic/profile-fields-questions