BuddyPress

I know using Welcome Pack I can send to new users invites to join some specific groups.

Is there a way to allow user to join groups while registering? Essentially, I’d like to list all groups as checkboxes on registration page so that users can select (check) groups and when they click “Register”, they automatically join selected groups.

Also, I will use hierarchical groups via BP Group Hierarchy, so I’d like to see a collapsible tree of groups on the registration page…

Not with Welcome Pack, I”m afraid.

As it turns out, there’s an app for that

http://wordpress.org/extend/plugins/buddypress-registration-groups-1/