groups_notification_new_membership_request( int $requesting_user_id, int $admin_id, int $group_id, int $membership_id )

function groups_notification_new_membership_request( $requesting_user_id = 0, $admin_id = 0, $group_id = 0, $membership_id = 0 ) {

	// Trigger a BuddyPress Notification.
	if ( bp_is_active( 'notifications' ) ) {
		bp_notifications_add_notification( array(
			'user_id'           => $admin_id,
			'item_id'           => $group_id,
			'secondary_item_id' => $requesting_user_id,
			'component_name'    => buddypress()->groups->id,
			'component_action'  => 'new_membership_request',
		) );
	}

	// Bail if member opted out of receiving this email.
	if ( 'no' === bp_get_user_meta( $admin_id, 'notification_groups_membership_request', true ) ) {
		return;
	}

	$unsubscribe_args = array(
		'user_id'           => $admin_id,
		'notification_type' => 'groups-membership-request',
	);

	$group = groups_get_group( $group_id );
	$args  = array(
		'tokens' => array(
			'admin.id'             => $admin_id,
			'group'                => $group,
			'group.name'           => $group->name,
			'group.id'             => $group_id,
			'group-requests.url'   => esc_url( bp_get_group_permalink( $group ) . 'admin/membership-requests' ),
			'membership.id'        => $membership_id,
			'profile.url'          => esc_url( bp_core_get_user_domain( $requesting_user_id ) ),
			'requesting-user.id'   => $requesting_user_id,
			'requesting-user.name' => bp_core_get_user_displayname( $requesting_user_id ),
			'unsubscribe'          => esc_url( bp_email_get_unsubscribe_link( $unsubscribe_args ) ),
		),
	);
	bp_send_email( 'groups-membership-request', (int) $admin_id, $args );
}