A: Beyond simply editting /etc/group with your favorite text editor, adding a user to the wheel group can be achieved in a few different ways.
To place a user in the wheel group when her account is first created, you would run
$ sudo pw user add jdoe -G wheel
Note: The above also creates the jdoe group automatically, use the -g argument if you wish her default group to be different.
If the user already exists and you wish to add her to the wheel group, you would run
$ sudo pw group mod wheel -m jdoe