This add-on by Scott Hair, allows you to select certain groups you would like new members to automatically be enrolled (join) in. The component will also join existing members to the selected groups as well. Great option if you have groups you would all members be involved in. Support group, or current events groups can greatly benefit by having all members enrolled in their group.
Creates a simple admin page that allows you what groups you wish to auto join.
Joins all new users to those selected groups.
Joins all existing users to those selected groups if they are not members and have not been banned.
Ideal for BuddyPress install with groups that everyone should be a member like a “Support” group.
Plugin homepage: http://twodeuces.com/wordpress-plugins/buddypress-auto-group-join