Automatically join any group when new user registers in Buddypress

function auto_join_group($user_id, $group_id) {
    if( !$user_id ) return false;

    // Join user to group with group id $group_id
    groups_accept_invite( $user_id, $group_id);
}

add_action( 'bp_core_activated_user', 'auto_join_group' );

Useful Buddypress Functions

bp_loggedin_user_id() = Get logged in user id
bp_displayed_user_id() = Get id of displayed user
bp_core_get_username($user_id) = Get username for the given id