If you FAdmin as well, that may cause some conflicts, but you can always go into the !menu and under the 'groups' tab, find 'head-admin'. Under 'user management' tab (on the left) there will be a 'ulx adduser' on the right. Click on it (not the checkbox, the actual text saying 'ulx adduser') and on the right it says 'restrict groups' (or something). In there you can set the whitelist (the groups that are allowed), you can type in the groups (case-sensitive) that the groups is allowed to add users to. Just make sure not to type 'superadmin' and 'Owner' in there.