There is a way to do this conversion:

PrimaryGroupID -> Group Name
Group Name -> PrimaryGroupID

Where PrimaryGroupID is the numerical value stored in USER ADSI object:

$obj = getobject("WinNT://$DOMAIN/Administrator,user")

? $obj.PrimaryGroupID

Returns:

1056

How can i convert this number to Group Name ?
And How can i convert group name to PrimaryGroupID?