MultiSite + bbPress (on sub-site) + user management = ???

I have a multisite setup using subdomains, latest versions of everything. I have bbPress installed. I have it active on a sub-site that is domain mapped. The forum looks nice, seems to be working fine.

Until someone wants to post.

They need to be registered.

If I provide the registration link, it takes them back to the primary site.

When they confirm their registration, it shows them as a WordPress subscriber, not a bbPress participant.

I've googled extensively and one "success" others had was to use an AJAX login/register sidebar widget plugin, but when I do that the user never gets their confirmation with password.

Does anyone know how to get streamlined user management working on bbPress running on a Multisite sub-site?