Changes to groups are allowed only in group edit mode. Use the Edit Group command to change to all instances of a group type. You may use the "Ungroup" option to proceed with this change by ungrouping the changed group instances. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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