edited from original question because the real problem was something unrelated to the question
I got a list of trustees from NTFS permissions and now I want to expand the groups to show membership. If I have a SAM name like MyDomain\name, there's no indication of whether that is a group or not. The Get-ADobject command has an ObjectClass property which will indicate group or user if this is an Active Directory domain object. One can use:
Get-ADObject -filter 'SamAccountName -eq "My Users"'
or
$sam = "My Users"
Get-ADObject -filter 'SamAccountName -eq $sam'
Thanks to JPBlanc who had an alternate form of writing that with a script block and some other suggestions.
And thanks, user2142466. That looks like a good suggestion for my original script.