Powershell Filter For Active Directory Groups Only.

I am trying to automate a network wide (about 2.4 million folders) search for those folders, what Active Directory groups have access to them, and what type of access that is.

I have code that does the above, but it does it for every item on the folders, except the ones that I filter out with code such as:
    $ACLs = Get-Acl $Folder.FullName |
    ForEach-Object {$_.Access} |
    Where {$_.IdentityReference -notlike "*BUILTIN*" -and $_.IdentityReference -notlike "*NT AUTHORITY*"}

There are more in my list of "Where" statements, and this works ok, but I would like to only show Groups, and not User accounts.

Some testing script I've started for checking one folder is as follows:
    (Get-Acl "\\Server\FilePath").Access |
    Select-Object FileSystemRights,AccessControlType,IdentityRefernece
    Where {$_.IdentityReference -notlike "*BUILTIN*" -and $_.IdentityReference -notlike "*NT AUTHORITY*"}

This gives me a list of Users and Groups, along with their access to that folder.

My question is: How do I filter out for only showing Groups for this kind of folder permission information gathering?