2013 – Remove all users who are not in a security group through Powershell

How can I remove all users who are NOT in a site collection security group through Powershell in Sharepoint Server 2013? Currently, I can remove all users in the default security groups (members, owners, visitors) with my script. However, I could not figure out how to remove users who are outside a group. However, I do not want a specific user with credentials removed. I want to remove all users who are not in a group.

Many Thanks.