I'm trying to surround the object called diatom in the center of the scene with a ring of debris. My original plan was to use a torus or similar piece of geometry to select the primitives surrounding the diatom and group them. Then I would scatter points over that group and copy platonic objects to them.

Unfortunately, you can only use geometry to create a point group. I tried to convert a point group to a primitive group, but I was unable to.

Any suggestions?

if you are talking about your object called “surface” then you couldn't convert point group to satisfying prim group because the grid is Mesh type object, it needs to be polygon
so just change primitive type on grid to polygon
then group points by object and convert group to primitive

this may be also interesting for you if you want to scatter points over surface

http://www.sidefx.com/index.php?option=com_content&task=view&id=1030&Itemid=216 [sidefx.com]
http://www.sidefx.com/index.php?option=com_content&task=view&id=1032&Itemid=216 [sidefx.com]
http://www.sidefx.com/index.php?option=com_content&task=view&id=1040&Itemid=216 [sidefx.com]

For a ring of debris, would'nt it be better to create a torus and re-size it to suite, and run that through an isooffset node as a fog volume, then scatter points with a scattersop based on that volume, rather than having all the points sat on a similar plane?

Even drop a measuresop and create an area attrib, then scatter the points based on that area….

…just a thought.