editusergroups-start.action does not check if a user actually exists:

If a non-existing username is passed as a parameter in the URL below:

http://<hostname>/admin/users/editusergroups-start.action?username=dummyuser

it does not give an error for the non-existing user, instead Confluence displays the message "User is not a member of any group" (screenshot is attached).
If any one of the available groups is selected and then click on the "Join>>" button, the user is redirected to this page instead:
http://<hostname>/admin/console.action

A check should be made for non-existing users and a message such as "User does not exist" should be displayed when the above URL (http://<hostname>/admin/users/editusergroups-start.action?username=dummyuser) is accessed.