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.