OK, then yes, totally possible to do with a script. There are a couple of methods to do it, but I believe the current supported way (still) is to use dseditgroup Something like this:
/usr/sbin/dseditgroup -o edit -a username admin
In the above, you are telling dseditgroup to do the operation (-o) edit to add (-a) the user username to the group admin
This makes the user named "username" an admin
To do the reverse - remove them from the admin group, you use operation delete (-d) like so:
/usr/sbin/dseditgroup -o edit -d username admin
This simply deletes "username" from the admin group.
Hope that helps.
