Yes. You can tag a user in a document through the discussions tab.

If the discussions tab is not visible when viewing a document, click the speech bubble icon in the top right corner. This will pull out the discussions area with a running list of all comments made regarding the current document.

To tag a user, simply start a new comment and type the at symbol, @. A list of users who have access to the document will appear. You can continue to type the user's name and it will narrow the list with each letter you type. When you see the user you wish to tag on the list, you can either click their name, or use the up and down arrow keys on the keyboard to highlight the desired user then hit tab on the keyboard to finish selecting their name.

You can tag before, after, or inline with your comment. You can also tag multiple users by individually adding each user to the comment with a new @ symbol each time.