I added some users by inviting them, they respond, are created and confirmed. They can login, all perfect. But they dont show in the list of users, so I cannot add them to an organisations or allow them to view Sammlungen.
I cannot find any Diagnose- menu
You also mentioned the users can login and the accounts are created correctly, I am confused how you are unable to add them to the organization. Do you get any errors when trying to add a user to the org? Do you happen to have SMTP set up?
It depends on how you invited them, via the admin panel, or org tab.
Via the org tab they should have received a mail with a token which is checked during account creation and first login. They may need to use that token again, or you need to re-invite them.
If you did it via the admin page, then you need to invite them to the org, inly then they will appear within the org tab.