Steps to reproduce the issue

Go to the Joomla user manager and click New to create a new user.
On the tab Account Details input all required information.
On the tab Assigned User Groups uncheck all groups, so no group is checked.
Click Save & Close.
The user is now created with no usergroup assigned to the account.

Expected result

When creating a new user at least one selected usergroup should be required.

Actual result

A user can be created with no usergroup assigned to the account.

System information (as much as possible)

Additional comments

This should not be possible. The result could be that the user receives an email with the account details and the user could even login into the website. But as the account is not assigned to any group, the user cannot do anything, possibly not even logout or edit the profile.