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.
hmmmm..... thanks for the answer, but unfortunately it doesn't work, have a look at the below screenshot, i can only tag people in the "internal review" stage but not any of the "markting tam" even though that stage is currently active:
From your screenshot, it would appear that your Internal Review stage is configured to be a private stage. Since tagging a user in a subsequent (non-private) stage would give them visibility of the private stage, it's likely that Workfront is blocking you from doing this.
Are you able to test this with no privacy settings on the stage and see if you still have this problem?