Revit does not allow grouping of some of the selected elements. These elements will not be included in the group.

public static FailureDefinitionId UnableToGroupElems { get; }

Public Shared ReadOnly Property UnableToGroupElems As FailureDefinitionId
	Get

public:
static property FailureDefinitionId^ UnableToGroupElems {
	FailureDefinitionId^ get ();
}