groups_reject_membership_request( int $membership_id, int $user_id, int $group_id )

function groups_reject_membership_request( $membership_id, $user_id = 0, $group_id = 0 ) {
	if ( !$membership = groups_delete_membership_request( $membership_id, $user_id, $group_id ) ) {
		return false;
	}

	/**
	 * Fires after a group membership request has been rejected.
	 *
	 * @since BuddyPress 1.0.0
	 *
	 * @param int  $user_id  ID of the user who rejected membership.
	 * @param int  $group_id ID of the group that was rejected membership to.
	 * @param bool $value    If membership was accepted.
	 */
	do_action( 'groups_membership_rejected', $membership->user_id, $membership->group_id, false );

	return true;
}