Assigning customer to non-default group causes no products to be displayed

Question

I don't know much about Magento but I've been tasked with writing a module at work. I've been testing it on a local Magento 1.8.1 install with almost completely default options. The time has come to move to integrating it with an existing Magento 1.8.0 setup. With this new setup, I'm encountering a problem that I never had in testing.

When I create a new customer group and assign customers to it, those customers then can't see any of the products that would be shown if they were in the default General group. For these custom group customers, I see no products in any category.

I've looked at all the likely options in the admin panel and searched around on the internet but I couldn't find anything. What could be the cause of the problem? Is it a config setting or a module they've installed or something else entirely?

Thanks, friends!