unable to group copy geom features in skeleton

The referenced document says:

• Historically import features and independent geometries cannot be grouped.
• In order for a feature to be grouped it is required to be possible to copy it and/or add it in a User Defined Feature. Import feature and independent geometries cannot be copied or added to UDF.
• in the case of External Copy Geometry the reference models must be in session

I didn't know there were these restrictions on what could be grouped, so I learned something new!

In my case, the copy geom features are indeed external copy geom features, but the reference model is in session when I try to group the features.  Perhaps it's because the referenced model is a part with assembly cuts (now that I think about it)?

I'm going to mark this as a correct answer because I suspect this is as close as I'm going to get.  Many thanks!  But if anyone else has any more insights, please share!