How to check the number of site collection users using PowerShell?
Solution:
Go to the server side, and open PowerShell, then enter the command line below:
$site=Get-SPSite("http://konglh2008r2")
$web=$site.RootWeb
$web.SiteUsers
Note: Please replace the bold part with desired site collection address.
Applies for: SharePoint 2013, SharePoint 2010, SharePoint 2007