I see now that there IS a way to add a user directly - from Site Admin | Users. Once a user is entered there, a blog owner can use their Users panel to invite that email address to become a user on that blog.
I guess I'm not seeing how this is useful - this puts the responsibility for adding users on the main administrator, rather than on individual blog owners (in my case, the teachers that will be running the various blogs).
So I don't want to enable public signups, and I don't want to create users myself. I want blog owners to be able to create users for their blogs, but it seems like WPMU is architected with the reverse workflow.
Suggestions welcome. Is there a plugin to make it possible for blog owners to add users directly? Anyone know why it doesn't work this way by default?