Can Users Register as Participants Only with Prosites?

I have enabled Prosites. Working fine. However, there doesn't seem to be a way for users to register as Participants without having to create their own blog. I want to give users an option to just register as Participants so they can participate in BB Forums across the entire network (Forums on the primary domain, and any subdomain Forums). I have enabled the option in Prosites to allow users to set up a free site (i.e. they can choose "No thanks" and by-pass the Pricing Table), but I can't find the option for users to register as Participants so they can start contributing to all forums within the network.

Thanks in advance for your help!