This may be too late to help you, and you may have already found a solution, but from what I've found on this, I believe only publiic elements are serialized.
So, if you remove the DesignerSerializationVisibility attribute on the private property, and apply it to the public ribbon group (not the private), this should solve your issue. Again, I may be wrong on this, but it's what I'm doing in my stuff and it seems to work.
Also, depending on the IDE that you're using, you may have issues with the designer code updating itself. In the past, I've had to edit both the .designer and .resx files to get them to remove the serialization so I can mark the object properly.
If you've found the solution to your issue, please post it here so we can see how you've done it.
Thanks,
Mark