I’ve found this article on sharing a user base between sites.
http://www.remicorson.com/share-users-database-on-different-wordpress-installs/
It seems to work, but where my main problem lies is logging in to the sub site.
Even when I don’t have the shared user base enabled, I still get redirected to the login page after logging in…
I am having the same exact problem after installing this plugin…
how did you clear your cache on the site?