In order for users to appear in the Tagging of a Proof they should meet one of the following criteria:
* should have opened the Proof at least once
* must be an Owner or a Reviewer of the Proof
* should be added in Advanced Workflow
You can see which users can do Tagging on a Proof by opening the People tab in the top left of the proof viewer.
To Tag a user you should click on "Add comment: button type "@" sign there, and the list of users that can be tagged will appear accordingly.