That's not really something Toolset can help you with, you'll need to write some custom code for that.
Depending on your requirements, it might make sense for you to record which posts a user has seen (i.e. store the data as user meta) or which users have seen a post (i.e. store the data as post meta).
I think the most reliable way to trigger your code, i.e. when someone visits a post, is to add your code directly to the template for displaying single posts.
If this refers to a custom post type, e.g. "Things", then you could duplicate the generic single.php and save a version single-thing.php which is used just for the thing post type. (You'll want to be doing this in a child theme.)
You'll need to decide the best format to record the post ids (if storing as user meta) or user ids (if storing as post meta), and that decision may depend on how and where you intend to display information about the number of read (or unread) posts, which are likely to involve writing custom queries.
The sidebar lists Toolset Contractors you could contact to do this for you, although as it is not specific to Toolset you should find any WordPress developer would be able to implement it.
