Not sure how I missed this in the past, or have not had this come up before, but here is something I just had to use today.
Comparing 2 AD Groups in Poweshell.
diff (Get-ADGroupMember "Group1") (Get-ADGroupMember "Group2") -Property 'SamAccountName' -IncludeEqual
You will get 2 columns:
User (== or <= or =>
== in both
<= in first group
=> in Second group