Problems in administrating user custom field rights


I have got a group G with the Users U1 and U2 and a user custom field
CF. That group is supposed to modify and see that custom field.
Nevertheless, if I login with U1 (not root!) , I can neither see nor
modify CF unless every privileged user is allowed to see and modify it.
You can test that, if you copy Admin/Users/Modify.html in a local
directory which is not in local/html/Admin/

Is there any way in which I can only allow users of G to modify and see CF?