Get-Member is not for getting user's group membership. If you want to get a list of groups a user belongs to on the local system, you can do so by:
$query = "ASSOCIATORS OF {Win32_Account.Name='DemoUser1',Domain='DomainName'} WHERE ResultRole=GroupComponent ResultClass=Win32_Account"
Get-WMIObject -Query $query | Select Name
In the above query, replace DemoUser1 with the username you want and the DomainName with either your local computer name or domain name.