How to add a user to the wheel group?

In Leopard, users can be added to system groups using ' dscl'. For example, while logged into an "admin" account, the command below can be entered using "/Applications" > "Utilities" > "Terminal.app" to add a user "username" to the "wheel" group:<pre>
sudo /usr/bin/dscl . -append /groups/wheel GroupMembership username</pre>

If you prefer a GUI, "Workgroup Manager.app", included with the Leopard version of the "Server Admin Tools" can be used.
http://www.apple.com/support/downloads/serveradmintools105.html

I wouldn't muck with the wheel group. It is used internally by OS X and modifying it could have unintended consequences.

If all you want is to be able to get to root on the command line from a non-admin account, that is easily accomplished by using su and then sudo:

$ su admin
Password:
bash-3.2$ sudo -s
Password:
root#

I would just edit the sudoers file and add your "user" name in there, and make it has access to everything:

user ALL = (ALL) ALL