Looking around the web it seems one of the most commonly requested features is to allow people to sign up at any multisite blog not just the main one, and without being redirected back to the main blog.
There are numerous ways to do this, including coding something into your theme, using the wp-signup file as a template or it gets more complex with BuddyPress in play.
It dawns on me though that your membership plugin is already doing some of what people are asking. When enabled the BuddyPress appears on a given page for people to sign up to.
So why not strip those parts out and make it into a small and lite plugin which will allow any site in a multisite install to have its own members sign up and get a role as set by the site admin or not as set by the super admin (ie only one default role allowed for security of your site admins not really knowing what they are doing with permissions).
I've seen this request has been made over and over throughout the years of WPMU. Some plugins are around, mostly out of date.
I like the idea of my site owners having their own registration form, user signs up, obviously registered at the main site with a default role at the sub site.