Find properties/methods of a class?

The exercise im trying to do is find the SID of each group on a local machine.

I want to do this without simply copying down a script. 
I know i need the System.Security Class, but i cant figure out how to see the methods/classes/properties in this class.

Ive tried 

System.Security | Get-member

and 

$newObj = System.Security
get-member $newObj

and a few variants. Can anyone advise how i explore the classes from within powershell?