Lack of Interest Assigning User Groups at Sign Up

We have recently launched phatollie.com and are using the member map add-on so that skate parks can add their location.
No, what I would like to be able to do is at the sign up stage, can I include a field whereby the visitor selects whether they are a "Skateboarder" or a "Skatepark" and then automatically get assigned to that user group, obviously we want this forum to be huge and dont want to have to assign user groups manually.
I'm sure this is something that is already "in the box" but I just cant find it
Any help would be appreciated xenforo'ers thanks.