groups_reject_invite( int $user_id, int $group_id, int $inviter_id = false )

function groups_reject_invite( $user_id, $group_id ) {
	if ( ! BP_Groups_Member::delete_invite( $user_id, $group_id ) )
		return false;

	/**
	 * Fires after a user rejects a group invitation.
	 *
	 * @since BuddyPress 1.0.0
	 *
	 * @param int $user_id  ID of the user rejecting the invite.
	 * @param int $group_id ID of the group being rejected.
	 */
	do_action( 'groups_reject_invite', $user_id, $group_id );

	return true;
}