groups_notification_membership_request_completed( int $requesting_user_id, int $group_id, bool $accepted = true )

function groups_notification_membership_request_completed( $requesting_user_id = 0, $group_id = 0, $accepted = true ) {

	// Trigger a BuddyPress Notification.
	if ( bp_is_active( 'notifications' ) ) {

		// What type of acknowledgement.
		$type = ! empty( $accepted ) ? 'membership_request_accepted' : 'membership_request_rejected';

		bp_notifications_add_notification( array(
			'user_id'           => $requesting_user_id,
			'item_id'           => $group_id,
			'component_name'    => buddypress()->groups->id,
			'component_action'  => $type,
		) );
	}

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

	$group = groups_get_group( $group_id );
	$args  = array(
		'tokens' => array(
			'group'              => $group,
			'group.id'           => $group_id,
			'group.name'         => $group->name,
			'group.url'          => esc_url( bp_get_group_permalink( $group ) ),
			'requesting-user.id' => $requesting_user_id,
		),
	);

	if ( ! empty( $accepted ) ) {

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

		$args['tokens']['unsubscribe'] = esc_url( bp_email_get_unsubscribe_link( $unsubscribe_args ) );

		bp_send_email( 'groups-membership-request-accepted', (int) $requesting_user_id, $args );

	} else {

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

		$args['tokens']['unsubscribe'] = esc_url( bp_email_get_unsubscribe_link( $unsubscribe_args ) );

		bp_send_email( 'groups-membership-request-rejected', (int) $requesting_user_id, $args );
	}
}