how to setup limit access to accounts and contacts per user- or user group

Hi there, here is a question I am not sure on how to set it up.

I want to add a user to our platform that can work with all the basic features but has limited access to our entire customer database. This means, he/she can only see the accounts and customers we put in a specified group.

How do I set this up? Or is there an example somewhere i can follow?