I just came across this scenario and thought I'd share it in case it saves you some time.
I have a fairly large solution in VS2017 and have a shared project that contains some common code such as enums.
I just tried to add my shared project to another project (right click project, add reference and go to shared Projects section) and noticed that the shared projects was empty. It wasn't giving me the shared project as an option.
See my answer below...