This .NET class wraps the AcDbGroup ObjectARX class. 

The Group class represents a collection of entities referred to by a single name. All Group objects belong to a dictionary object which can be obtained through the Database.GetGroupDictionary() method. 

Unlike a block, entities within a group can be individually manipulated. 

This class contains a dynamic array of object IDs that are the objects in the group.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Group

Public Class Group
Inherits DBObject

public class Group : DBObject;

Group plants persistent reactors on its entries when the entries are added to the group. To accomplish this, it must have an objectId. When the group is added to the group dictionary, it is also added to the database, and thus it obtains an objectId. Therefore, do not attempt to add entries to a newly created group before adding the group to the group dictionary.

GroupIterator, Entity, ObjectId