BackPress – which libraries i need to load to work with Multisite?

BackPress does not offer any multisite support libraries, you’ll have to build those yourself with the support of the WordPress multisite source code.

The following are the major parts making up multisite functionality:

As you can imagine, it’s going to be quite hard to bring it all together, but everything is pretty much there, with additional bits and pieces scattered across other files. By the way, the old MU project http://mu.trac.wordpress.org/browser/trunk is also inside of a WordPress environment, so it doesn’t give any advantage; you’ll still have to gather the functionality together piece by piece.

Unless you have a very good reason to use BackPress, I’d stick with WordPress instead, a lot can be done with must-use plugins in terms of providing a standalone application package.

Hope this helps.