BuddyPress.org

if you want to keep it all using bp functions, look at bp-groups.php

I would set it up hooking on activating the user (or signup if you don’t care about the activation key)

_action( 'bp_core_activated_user'

then call (with no pagination passed in and active type)

groups_get_groups(

then loop over all the groups and call (which needs the user_id and group_id)

groups_join_group(

only downside to join_group is it will call a new activity for joining – but a plus side is it updates the groupmeta counts.

*my standard disclaimer applies – only in theory nothing tested.