3.1.1.4.5.11 primaryGroupToken
Let TO be the object from which the primaryGroupToken attribute is being read.
The value of TO!primaryGroupToken is the RID from TO!objectSid when there exists C in TO!objectClass such that C is the group class. Otherwise, no value is returned. That is, if TO is a group, then the value of this attribute is the RID from the group's SID. If TO is not a group, no value is returned when this attribute is read from TO.