When a logged in user visits a blog for which they have no capabilities (i.e. not subscriber), they are automatically added as a subscriber.
*** PLUGIN DEVELOPMENT ABANDONED - It is no longer useful and/or compatible with recent versions of WPMU or WP >= 3.0 ***
Mu-ABS (which stands for Mu Automatic Blog Subscription) is a mu-plugin for WordPress Mu. When you build a blog network with such a platform, you probably wish that a user subscribed to one of your blogs is actually subscribed to each blog of the network. This does not natively happen with WordPress Mu. This mu-plugin changes this behavior. When logged-in users visits a blog for which they have no capabilities (i.e. not subscriber), they are automatically and transparently added as a subscriber.