Hi Philip,
Since tagging another user into a proof will grant access to the proof, the users that can be tagged are only the people that are part of that proof Workflow. This is to protect the integrity of the proof / Workflow. There are some exceptions to this though, if you have a role of Autor or Moderator on the proof, or you are the proof owner, you are able to tag others as you have enhanved priviledges to that proof. Users that have the Supervisor or Administator proof permission profile also have the ability to tag others.
This article here explains more.
Best Regards,
Rich.