Just to clarify some comments made above in case any new Bryce users stumble in here, the use of a negative is not truly needed to create a boolean mesh for export. Another Positive would work as well. You just need to make sure that all objects before you group them are either set with a positive or a negative atttribute. A neutral attribute on an object in Bryce will not allow you to make a boolean mesh.

In other words, 2 positive objects can be grouped together and then converted to a boolean. If you then want to add to that, you can set your boolean to positive and then add another negative or positive object to it and once grouped it can also be turned into a boolean for export. Just make sure all object attributes are set before grouping as if you create a boolean but have missed setting the attributes on some item in the mix, this item will not show up if you export the final item as a OBJ file, etc.  ( I have learned that the hard way, way too many times and it can really become an issue when making complicated models with several booleans added to each other)

Bruce