It would be great if usernames for fields like Reviewers and CC could be via a multi-select pop-up prompt. Similar to Bugzilla when you configure the usemenuforusers option.
I suspect it may take away some usability for when people want to enter a users email who does not yet have an account. So, here is an idea.
You could leave the existing field as is, but have the Topic entry screen also have two multi-select popups, one for Reviewers and one for CC. The selection entries would be from the usertable. Then after submit, add any selected users into the existing plain text field.
The problem with the existing entry is that it's too easy to typo on an email address or just get it totally wrong.