Can I allow a subscriber on a subsite to signup for their own subsite with their existing account?

I want to allow a subscriber of subsite.mysite.com to register for their own subsite using their existing user account. I'm using Pro Sites. Is this possible?

Right now, when a subsite subscriber goes to my Pro Sites signup page while they're logged in, they're met with a message that says they aren't an administrator of any sites and they can't create their own.