@Peter Teunissen Unfortunately, this is not a supported use-case.
There is a quick and dirty workaround, though:
You can modify the existing user's e-mail address in the UAA db with the following command to avoid the collision:
docker exec -it cbreak_uaadb_1 bash
psql -U postgres
UPDATE users SET username = '<EMAIL>' where username = '<EMAIL>';
After these changes, you should be able to invite the user from Cloudbreak UI.
Hope this helps!