root and subdirectories question (forcing directory)

I just installed WP and enabled the network.

The “root” of the site is www, and it is set to subdirectories – NOT subdomains.

I want the root of the site to of course stay at the regular http://www.mysite.com, however, I am setting this up to display ‘demos’, so I would really like to have the multi sites (“users/sites”) to appear as:

http://www.mysite.com/demos/(site/username here) such as:

http://www.mysite.com/demos/businessdemo
http://www.mysite.com/demos/homedemo
http://www.mysite.com/demos/whatever1
http://www.mysite.com/demos/housedemo
http://www.mysite.com/demos/anotherdemo

Obviously I recognize that the last part will be whatever I set it to when I create the “blog name”, but is there a way of setting the defaults for USERS to be under the folder /demos/

or do I have to make that a subdomain and install there? (But then I’d have two installs…. defeating the purppose).

Thoughts?