This a snippet to auto join "just activated" members in one or more BuddyPress groups.

bp-custom.php

<?php


if ( ! class_exists( 'Imath_Auto_Join_Groups' ) ) :
/**
 * AutoJoin new members to chosen groups
 *
 * This is a little snippet, feel free to : 
 * - use it
 * - extend it..
 * - copy it...
 * - build a plugin out of it :)
 * 
 * But sorry, i have no time to support it :(
 */
class Imath_Auto_Join_Groups {
	/**
	 * Start the class.
	 *
	 * @access public
	 *
	 * @uses buddypress() to get BuddyPress main instance.
	 * @static
	 */
	public static function start_groups_autojoin() {
		// Bail if groups component is not active
		if ( ! bp_is_active( 'groups' ) ) {
			return false;
		}

		$bp = buddypress();

		if ( empty( $bp->groups->imath_aj ) ) {
			$bp->groups->imath_aj = new self;
		}

		return $bp->groups->imath_aj;
	}

	/**
	 * Construct.
	 *
	 * @access public
	 */
	public function __construct() {
		$this->setup_globals();
		$this->setup_hooks();
	}

	/**
	 * Set Globals
	 * 
	 * Using public here so that filters can be reached
	 * 
	 * @access public
	 */
	public function setup_globals() {
		// What are the auto joinable groups ?
		$this->group_ids = bp_get_option( 'imath_aj_group_ids', array() );

		// Use the filter or directly edit in favor of your language..
		// Sorry, this is a quick snippet so i won't handle internationalisation.
		$this->bulk_label_set   = apply_filters( 'imath_aj_bulk_label_set',   __( 'Set as Auto-join', 'imath-aj' )   );
		$this->bulk_label_unset = apply_filters( 'imath_aj_bulk_label_unset', __( 'Unset as Auto-join', 'imath-aj' ) );
		$this->status_label     = apply_filters( 'imath_aj_status_label',     __( 'Auto-join', 'imath-aj' )          );
	}

	/**
	 * Define hooks ( actions & filters )
	 * 
	 * Using public here so that filters can be reached
	 * 
	 * @access private
	 */
	private function setup_hooks() {

		/** "Create" the UI to define the autojoin groups *****************************/

		/**
		 * We will use the Groups Admin Panel within WordPress Administration
		 * 
		 * > add a new bulk actions to available ones (by default BuddyPress only have the bulk delete one)
		 * > add an info near the group visibity status (is auto join ?)
		 */

		// Add a bulk action in the groups admin panel to define auto joinable groups.
		add_filter( 'bp_groups_list_table_get_bulk_actions', array( $this, 'bulk_auto_join' ), 10, 1 );

		// Should probably create a new WP List Table column, but, hey! This is a quick snippet ;)
		// So i'll use the visibility status column of the groups admin panel
		add_filter( 'bp_groups_admin_get_group_status', array( $this, 'auto_join_status'), 10, 2 );

		// Process autojoin / unautojoin
		add_action( 'bp_groups_admin_load', array( $this, 'handle_bulk_auto_join' ), 10, 1 );


		/** Auto join new members *****************************************************/

		// Catch user just after activation
		add_action( 'bp_core_activated_user', array( $this, 'autojoin_user' ), 11, 1 );
	}

	/**
	 * New bulk actions
	 * 
	 * @access public
	 * 
	 * @param  array  $bulk_actions list of available bulk actions
	 * @return array                same list with our actions added
	 */
	public function bulk_auto_join( $bulk_actions = array() ) {
		return array_merge( $bulk_actions, array(
			'autojoin'   => $this->bulk_label_set,
			'unautojoin' => $this->bulk_label_unset
		) );
	}

	/**
	 * Info to inform the "row" group is autojoin
	 * 
	 * @access public
	 * 
	 * @param  string  $group_status visibility of the group
	 * @param  array   $group  the row group
	 * @return string                same visibility with our autojoin info added
	 */
	public function auto_join_status( $group_status = '', $group = array() ) {
		if ( ! empty( $group['id'] ) && in_array( $group['id'], $this->group_ids ) ) {
			$group_status .= '<br/><span class="attention">' . esc_html( $this->status_label ) . '</span>';
		}
		

		return $group_status;
	}

	/**
	 * Process the bulk actions and gives a feedback to admins
	 * 
	 * @access public
	 * 
	 * @param  string  $do_action the bulk actions beeing requested
	 */
	public function handle_bulk_auto_join( $do_action = '' ) {

		/** Admin Feedback ************************************************************/

		// First, let's check the url to see if bulk have been processed
		if (  ! empty( $_GET['updated'] ) && 'aj' == $_GET['updated'] ) {
			// Get the result message
			$feedback = (array) get_transient( '_imath_aj_feedback' );

			// This should never happen..
		    if ( empty( $feedback ) ) {
				return;
			}

			// Delete the feedback transient
			delete_transient( '_imath_aj_feedback' );

			// Init the notice
			$admin_notice = '';

			$feedback_type = array_keys( $feedback );

			// Build the message
			// arhg sorry for internationalisation :(
			switch ( $feedback_type[0] ) {
				case 'autojoin' :
					$admin_notice = sprintf( __( 'Number of groups added to autojoins: %s', 'imath-aj' ), intval( $feedback['autojoin'] ) );
					break;

				case 'unautojoin' :
					$admin_notice = sprintf( __( 'Number of groups removed from autojoins: %s', 'imath-aj' ), intval( $feedback['unautojoin'] ) );
					break;
				
				case 'error' :
				default:
					$admin_notice = __( 'Something went wrong, please try again later.', 'imath-aj' );
					break;
			}

			// Using bp_core_add_admin_notice() is convenient ;)
			if ( ! empty( $admin_notice ) ) {
				bp_core_add_admin_notice( $admin_notice );
			}
		}

		/** Bulk actions **************************************************************/

		// Bail if not our targetted bulk actions or no group to auto join
		if ( ! in_array( $do_action, array( 'autojoin', 'unautojoin' ) )  || empty( $_GET['gid'] ) ) {
			return;
		}

		// Global bulk nonce check ( "bulk-" . plural argument of WP_Liste_Table item )
		check_admin_referer( 'bulk-groups' );

		// Build redirect url
		$admin_url = admin_url( 'admin.php' );

		// BuddyPress is network activated ?
		if ( bp_core_do_network_admin() ) {
			$admin_url = network_admin_url( 'admin.php' );
		}

		$redirect = add_query_arg( array( 
			'page'    => 'bp-groups',
			'updated' => 'aj'
		), $admin_url );

		// Init the feedback message type
		$feedback_message = array( 'error' => 1 );

		// Get the group ids
		$group_ids = wp_parse_id_list( $_GET['gid'] );

		// Process
		if ( ! empty( $group_ids ) ) {

			foreach ( (array) $group_ids as $key => $group_id ) {
				// Already in & autojoin action : no need to add the group
				// unset to have the correct count
				if ( in_array( $group_id, $this->group_ids ) && 'autojoin' == $do_action ) {
					unset( $group_ids[ $key ] );
				}

				// Not in & unautojoin action : no need to remove from the group
				if ( ! in_array( $group_id, $this->group_ids ) && 'unautojoin' == $do_action ) {
					unset( $group_ids[ $key ] );
				}
			}

			// Prepare the updated autojoin groups
			$groups = array();

			if ( 'autojoin' == $do_action ) {
				$groups = array_merge( $this->group_ids, $group_ids );
			} else {
				$groups = array_diff( $this->group_ids, $group_ids );
			}

			if ( bp_update_option( 'imath_aj_group_ids', $groups ) ) {
				$feedback_message = array( $do_action => count( $group_ids ) );
			} else {
				$feedback_message = array( 'error' => $do_action );
			}
		}

		// Set a transient for the feedback message
		set_transient( '_imath_aj_feedback', $feedback_message, 30 );

		// Finally safely redirect the admin
		bp_core_redirect( $redirect );
	}


	/**
	 * Autojoin new member to the groups
	 * 
	 * @access public
	 * 
	 * @param  int  $user_id the just activated user id
	 */
	public function autojoin_user( $user_id = 0 ) {
		// YeeeeAH no user stop!
		if ( empty( $user_id ) ) {
			return;
		}

		// UhhOuu no autojoin stop!
		if ( empty( $this->group_ids ) ) {
			return;
		}

		// Before we go.. let's check the groups still exist...
		$groups = groups_get_groups( array(
			'include'     => (array) $this->group_ids,
			'show_hidden' => true,
			'per_page'    => false,
		) );

		// Dang autojoin groups have disappeared!
		if ( empty( $groups['groups'] ) ) {
			return;
		}

		// Now we can safely add the user to groups
		foreach ( $groups['groups'] as $group ) {
			groups_join_group( $group->id, $user_id );
		}
	}
}
endif; // class_exists check

// Load the auto join class
add_action( 'bp_init', array( 'Imath_Auto_Join_Groups', 'start_groups_autojoin' ) );

doesn't work.. :(