In previous WPMU installations, for any blog, the Users page listed all the users then had a section called "Add User From Community" which only allowed users to be added from the community.
This seems to have been replaced with the "Add New User" page. The problem I have with that is that the new page allows them to either add from the community or create a new user entirely.
I would prefer that blog admins just be able to add from the community (as in 2.6). It also complicates things that a blog admin has to type both the user's user name and email address to add them.
Interestingly enough, wordpress.com seems to still have the older 2.6 functionality.