Multisite, multi install logins

I am setting up a WP multisite network. I would like a way to allow a user's login to provide access to multiple installs within the network. But I need to be able to assign which installs for each user account.

Each user login will provide access to their WP install, but I also want to provide a link in the admin that redirects them to a site within the network I have set-up with a LMS/Buddypress. So I want all users account to be also logged into this site.

Is there a plugin that will manage this? If not can you point me in a direction that may help me solve this challenge?

- Dave