I was working with a customer last week who wanted an easy way to see all users (internal and external) who had access to their SharePoint site. There are several ways to do this – but the easiest way to see all users is by following the process below. This does not show which security groups the users belong to, it just shows what users have access to the site.
At the Site Collection level go to Settings->Site Settings:
Then go to People and Groups
You will get dropped out to the Group Membership page – change the number on the end to 0. (In the example below, it’s defaulting to 8.)
Once you change this to zero – you can easily see the names of every user who has access to this site collection.