Is there a limit of tags to associate with content?
It's a simple you must follow this 2 Steps :

First Step :


Create a views that will list your users that you want to reference
Add a Reference display to your view make your filters then Save


Second Step : 


Download entity reference module
Add new entity reference field  "Users that will access this content"
Chose the target type : User
Choose the mode "Views: Filter by an entity reference view"
Select the views that you've build on the first Step (the display type must be Reference and not page or block)


Now you will be able to select the users like a tags on content type but without any access permission

To restrict the Permission to the referenced user :
download the user reference access module 

Then configure it under the user entity reference field settings of your content type