When tagging a user in a comment, they must have campaign access. Without access, an error message will appear. The user must be added to the campaign audience to gain campaign permissions.
An error message will appear when tagging a user without campaign access.
Shown below is the error message.
To allow the user access to the campaign, you must add them to the campaign audience through the Campaign Management menu.
For more information on User Tagging, please visit: