I figured out how to do the grouping in Poser. I spawned props for the body parts covered by the T-Shirt, and set them to Hidden Line Display Style. Then hid all the parts, except the one that corrisponded to the group I was working on. The Hidden Line shows through in the Grouping Tool, so I could see where I needed to make the group.

The grouping is now done, though I'm not saying it is done well. That is yet to be seen. I have started work on the rig.