Assign member to usergroup at registration (tickbox)

[rob30UK]

I want to give users an option at registration to tick a box to automatically be assigned to usergroup X.

The tickbox will appear on the registration page and ask 'Automatically make me a member of the 'X' group?

If this box is ticked, they will get added to that usergroup when processing the registration.

Does this mod exist, or is someone prepared to make it..... or worst comes to worst, where in hell do i start?