To add an existing project to the solution, you simply right-click on the solution in the Solution Explorer window and select "Add-->Existing Project" and then browse to the osprj file. This will not make a copy of the project but will instead reference it from wherever it is located, so if it belongs to multiple solutions, any of them can edit it.
1. adding a project into a solution doesn't import the project (i.e. create a copy of it) but creates a link to the original project - changes to the imported project update the original project on disk;
2. adding an existing item (i.e. adding an automation) into a project appears to actually import it (i.e. create a copy) - changes to that automation don't affect the original automation in the original location.
Is this understanding correct, or did I use it wrong?
Thank you for the confirmation - it seems all the answers were in the previous responses but I hadn't been reading them properly, or wasn't aware of the nuances between projects and items when I first read them.