The setup used here was MacOS X 10.4 (not MacOS X Server) on a PowerPC MacMini.
The wheel group is already set up, but is not called wheel. The group wheel does exist, but the group admin is used by su as the wheel group. The user root belongs to both the wheel group and the admin group.
Another point to remember is that the system uses the NetInfo database, not /etc/group. When NetInfo Manager starts, it presents a list of items (like a list of folders). Select group, then in the next pane, select admin. In the window pane below, look at the property labeled “users” and see that your user id is there as well as root.
If you want to add another user to the “wheel” group (in actuality, the admin group here), add a new value to the users property. First, click the lock at the bottom right and enter your password so you can make changes. Select the users property. Next, in the menu bar, select Directory, and under that, select Insert Value. Put the selected user in the entry box that shows up and press Enter when done.
Don’t forget to save this or no changes will take place. This can be done with the usual Command-S or under the menu Domain, select Save Changes.