please forgive me if I got something wrong, but I still believe that my post is correct in the given context:
my aim is to create a list of domains belonging to a certain client. So I would fetch the client first by client_get() or client_get_by_username().
But then I only get the sys_groupid of the client, which is, as you pointed out, the id of the admin or the reseller.
Now I would need to send a request like sites_web_domain_get($session_id, array('sys_groupid' => $id))
and here I need the group_id of the user, which I do not have at this point.
So I'd use the client_get_groupid($session_id, $client_id) function.
Did I get something wrong? Is there a better way?