you don't need to add a user to wheel if a user does not need full su (root) power. If you want a useer to have FULL root rights then yess you add them to wheel (like YOUR own self)
If you desire a user to have limited root access then you use sudo.
You really don't have to add the user to any group but the default (user) which is done automagickly when adding a user and hitting the enter key. That is of course if you are only running a vanilla system.
Servers with multi-user/multi-net enviroments may have custom groups as in sales, production, admin, staff--- ETC ETC etc....
Them only certain groups need access to certain directories/services and are assigned accordingly.
I know that may be way more advanced than the original adduser question. But if this person is tring to set up a system with several users they may desire the knowledge to use other group and group setting for there users...
If not then they need to read the install instructions again about setting themselves up a user profile and that they should at least add themselves to the wheel group also so that they can have su (root) access in a console from their fav GUI. Unless of course they desire to run the box as root themselves...
sidebar note:
have built and installed 2 Slackware servers at work, samba and dns. Next comes the mail and webhost (covers 4 plants US wide). Already have contract for another Slack powered network for a new upstart factory in North Carolina... You may chose to disregard any post you wish cause you never know how advanced a "user" may be.