[dstruct2k]

Although it is possible to modify this code to assign a usergroup based off the content of a hidden profile field (which would achieve what you're looking for) I would suggest you rethink the layout of your forums if you believe people will find it "overwhelming" when they join.

If you force people to willingly block themselves from half of your site when they register, you'll either end up with everyone that's interested in the other half of the site making a second account, or a lot of PMs complaining that they want access to the whole site, not just part of it.

Perhaps your dilemma could be solved with a simple reorganization of the site