WordPress MU

I am going to migrate 5 WP blogs to WPMU, as it is difficult to manage that many different blogs (updates, maintenance, ect..) and I have several more that I may be managing in the near future. I am new to WPMU and have a questions about how to make sure all the register users of the blogs get imported into WPMU.

Note: This is really only an issue with two existing blogs (others are low volume sites).

Going through the XML export of one of the blogs, I can not see how register users are "imported" into the db. Is there some way to import the registered users? I don't want to ask several hundred users to "re-register".

Could this be done with multiple DBs or is there an "advanced" import function I am just not finding? Documentation would be worderful, if it exists.

Same situation here, except here it's about a dozen WP blogs. And I haven't seen anybody else comment, so I'm hoping a fresh comment will inspire a response.

This is the hard part. :) The export file only contains enough user info to assign an author to a post. It will create a user account, but without an email address (because it didn't pull it) and without a password (same thing).

import the user account into the MU database first. Yes, if you are merging multiple blogs, you will have to renumber user IDs and may attention to the blog ID they have permissiosn on. It is messy work. No, there is no guide or write up (yet - when I get around to it). Then, you import the posts.

this may help:
http://bavatuesdays.com/importing-a-single-wp-blog-to-a-wpmu-installation/