groups_send_membership_request( array|string $args = array() )

function groups_send_membership_request( $requesting_user_id, $group_id ) {

	// Prevent duplicate requests.
	if ( groups_check_for_membership_request( $requesting_user_id, $group_id ) )
		return false;

	// Check if the user is already a member or is banned.
	if ( groups_is_user_member( $requesting_user_id, $group_id ) || groups_is_user_banned( $requesting_user_id, $group_id ) )
		return false;

	// Check if the user is already invited - if so, simply accept invite.
	if ( groups_check_user_has_invite( $requesting_user_id, $group_id ) ) {
		groups_accept_invite( $requesting_user_id, $group_id );
		return true;
	} else {

		if ( true === bp_member_type_enable_disable() && true === bp_disable_group_type_creation() ) {

			$group_type = bp_groups_get_group_type( $group_id );

			$group_type_id = bp_get_group_type_post_id( $group_type );

			$get_selected_member_type_join = get_post_meta( $group_type_id, '_bp_group_type_enabled_member_type_join', true );

			$get_requesting_user_member_type = bp_get_member_type( $requesting_user_id );

			if ( in_array( $get_requesting_user_member_type, $get_selected_member_type_join ) ) {
				groups_accept_invite( $requesting_user_id, $group_id );
				return true;
			}

		}
	}

	$requesting_user                = new BP_Groups_Member;
	$requesting_user->group_id      = $group_id;
	$requesting_user->user_id       = $requesting_user_id;
	$requesting_user->inviter_id    = 0;
	$requesting_user->is_admin      = 0;
	$requesting_user->user_title    = '';
	$requesting_user->date_modified = bp_core_current_time();
	$requesting_user->is_confirmed  = 0;
	$requesting_user->comments      = isset( $_POST['group-request-membership-comments'] ) ? $_POST['group-request-membership-comments'] : '';

	if ( $requesting_user->save() ) {
		$admins = groups_get_group_admins( $group_id );

		// Saved okay, now send the email notification.
		for ( $i = 0, $count = count( $admins ); $i < $count; ++$i )
			groups_notification_new_membership_request( $requesting_user_id, $admins[$i]->user_id, $group_id, $requesting_user->id );

		/**
		 * Fires after the creation of a new membership request.
		 *
		 * @since BuddyPress 1.0.0
		 *
		 * @param int   $requesting_user_id  ID of the user requesting membership.
		 * @param array $admins              Array of group admins.
		 * @param int   $group_id            ID of the group being requested to.
		 * @param int   $requesting_user->id ID of the membership.
		 */
		do_action( 'groups_membership_requested', $requesting_user_id, $admins, $group_id, $requesting_user->id );

		return true;
	}

	return false;
}