I know there's a way to do it because way 'back in the day' I configured another cart to disable the 'add to cart' function until they've created an account.
http://www.srls.lucajo.com <--- example
... but I can't remember where you adjust it from. I have a different client that wants to make it so it does something similar (alternately disable the ability to view the catalog without an account completely..) but, again, I can't remember where to go to configure it.
Much thanks in advance,