bbPress EDU Theme integration with WPMU

I'm using the bbPress Edublogs theme (together with the bbPress Integration plugin with my WPMU install), but I've noticed for users to be able to post to the forum, they must also be at least a "subscriber" to my main blog on my WPMU install.

For whatever reason, they can be logged in to WPMU, but if they're not a subscriber (or higher) on that main blog, they won't see the box to "add new post" or "reply to post" in the forums.

Furthermore, when new users sign up on my WPMU install, they are not getting added to the main blog as a subscriber, even though I've set it in WPMU Site Admin > Options.

I checked in the bbPress Integration settings page, and I'm thinking the manual cookie settings may not be correct.

For example, the plugin suggests these:
define( 'COOKIEHASH', 'my-cookie-hash-code' );
define( 'COOKIE_DOMAIN', '.my-domain.com' );
define( 'SITECOOKIEPATH', '/' );
define( 'COOKIEPATH', '/' );

...but I recall from other posts on the forum that I shouldn't set these.

Just for reference, I have the COOKIEHASH set as suggested, and my SITECOOKIEPATH and COOKIEPATH are both set to null ('') instead of slash ('/').

But is that even the issue?