1. Navigate to Constituent360 > Groups
  2. In the text field, enter "Any Registered User"
  3. Click Search
  4. Click on the name of the group in the results returned (it's a blue hyperlink)
  5. Click Refresh Summary
  6. In the Group Summary section, look to the The number of users in this group
If you'd like to know how many of these constituents have valid email addresses, look to the The number of users with valid email.