As an Admin user, visit:
Admin > Users
Then run an empty search to list all users (i.e. just click 'Find Users' without adding in any search criteria).
This will return a list of all registered user accounts.
Finally, if you scroll to the bottom of this list you should be able to click the 'Export these users >>' link to get a copy into a file you can open in Excel. This makes it easy to see the total number of users registered for example.
From the excel spreadsheet, you can view the 'Last Logged In' column to help manage your users and delete any that have not logged in recently.