Kevin Holman starts his post out indicating this is a common request, and I’d have to agree. There are community solutions to this out there, but Kevin does (as always) a great job of explaining it, giving a walk-through, and provides some pretty screen captures!
Why would you want to create a NOT member group? From Kevin:
Suppose you have the following scenario: You are monitoring 1000 Windows Server with OpsMgr. In your management group, you have 100 servers that are Test/Dev machines, and you have 900 that are production. You need a simple way to treat these servers differently, for overrides, and creating notifications or incidents, or even scoping your views. You want to ensure that you don’t send critical pages, emails, or create incidents on these lab/test/dev machines.