What is the Users report in Piwik PRO Analytics module about?
In Piwik PRO Tag Manager you can provide an automatic setup of UserID. You only need to create a variable, which will be catching the userID value. The value can be anything you like, however, it is important it is unique, to avoid confusing visitors. Keeping that in mind, you can chose i.e. the email address of logged in user. To see how to set it up, take a look at our article on User ID.
The reports about UserID will be shown in the Users module in Analytics.