Get-ADGroupMember -Identity $GroupGUID |
ForEach-Object{
    if($_.ObjectClass -eq 'User'){
        # process user object
        $_ | Get-ADUser -Properties DisplayName
    }elseif ($_.ObjectClass -eq 'User'){
        # process group
        $_ | Get-ADGroup -Properties Name
    }
}

MemberName          ObjectGUID
----------------    ------------------
Test Group          123-456-XXXX
Test User           789-456-XXXX