Organic Groups Permissions Module?

Hello,

I'm wondering if there's a module available to only allow certain roles the ability to join an organic group.

For example, I don't want to give anonymous or authorized users the ability to join a group, only users of a certain role (or roles) are allowed to join a group.

I checked the modules page, but didn't see anything that seemed to fit. Does it not exist?

Thanks in advance,
-mike