Hey Hans,
it should actually add all the users. Make sure to disable error reporting in your global config - it's possible that some warning in the ajax response kills the script.

We are working on the new version that uses the Mailchimp's API v3 (as the version that we use now will no longer work by the end of the year) and we should release it by next week. That's why I don't want to invest time in the current issue as with the new version most probably it won't be an issue anymore. Let's continue this discussion if it's not working with the new version.

Regards,
Daniel