Adding local users from domain as local PC admin(?)

We have a room of PC's dedicated to one task which is used by
researchers here at the Eye institute where I work.

Often, in medical research facilities, there is software which is
brought in for legitimate purposes from different researchers. Its
much less formally managed than say a law firm or something.
I want to delegate the responsibility (on their request) of
administrator of these machines, to the professor/supervisor of these

These machines are XP, joined to a domain. We 'manage' this through AD
console. Its a 2000 server (not really important)
As I suggest in the subject , is there a way to ...add local users from
domain as local PC administrators?

I have attempted to do this by adding the Computer object to a Group I
have created, 'Grading-Machine Admin'.
Then, adding the User object to 'Grading-Machine Admin' group. This
did not work.

I don't really understand what exactly the relationships are between
the objects and resource, groups etc.
From what I can see you normally have a Resource that belongs to a
Group. A User can also belong to that Group. I'm sure its a one to
one, one to -many, many-many type issue that my brain can't comprehend.
... So I'd appreciated any one willing to empart their knowledge!!!