Returns the value of IsGroupedBy attribute. This attribute represents reference to the relationship IfcRelAssignsToGroup that assigns the one to many group members to the IfcGroup object.

IFC4 CHANGE: The cardinality has been changed from 1..1 to 0..? in order to allow the exchange of a group concept without having already group members assigned. It now also allows the use of many instances of IfcRelAssignsToGroup to assign the group members. The change has been done with upward compatibility for file based exchange.