We perhaps stumbled on an odd Autodesk user account/BIM360 bug.
We have a new user coming onboard next week. When we tried to add the user in https://manage.autodesk.com > User Management by User ... the process failed with "users weren't added due to network problems" error.
We found that BIM Manager invited the user to a BIM360 Project before we could assign him access to AEC Collection and BIM Collaboration Pro entitlements. We found the resulting email for the BIM360 project in the new user's MS Outlook inbox.
Somehow, although I don't clearly know how we could do this... we were able assign the entitlements even though we could not send him an invite in User Management by User.