We are unable to assign a bug to a specific person. The user account 'brad' can not be set in this text box, nor can 'Brad'.
The dropdown works when changing the attirbute from the administration panel.
But as its a feature out of the box it should atleast accept valid usernames (only 2 people on our webissues at the moment).
Oddly enough, after removing it, then re-adding it, it works as expected (found while writing this post). Only thing I had changed to how it was originally setup was I made it a required field. Error with default setup maybe?
If you create a new custom issue type of modify existing one, you can check "Allow selecting only project members". If you do so, and you currently have no members in that project you will not be able to choose any one.
So as far as I understand, you have been trying to assign a user to an issue which can be assigned only to members of project, which has no members.