This is where roles and permissions come in. Everyone who registers will belong or not belong to a range of your roles. You might have a role for receiving newsletters, one for people who have paid you something, one for people who can use a certain module and so on. Roles cna be assigned automatically, manually, or by a module when the person does something such as buys a product. You have to think in advance about all the roles that you might need for your audience.

You apply permissions for the roles to all your pages and modules and even the file system, depending on what people are supposed to be able to do.

I'd check out Smart Thinker's Events module for handling the registration for  the podcasts.You'd probably find the Gourps and Profile module hand as well, but just note that it takes some effort to find out the right configuration and you should get it sorted out well before getting people to sign up. It's much harder to change your heirarchy after the fact.

Rob