I did some work on this also, and it's much more complicated than it
seems at first. You can see all the work I did on this by going to
https://github.com/jokajak/clipperz-password-manager/ and looking at the
different branches that I have there.
The first step for me was to create the concept of contacts which are
other clipperz users. Once there you can encrypt the key for a specific
card and share it with another user. The problem is that you have now
granted complete access to that card because the key is used for both
encryption and authentication.
To really support shared cards you would need to find a way to separate
encryption and authentication so that a share could be read-only.
Hope this helps you get started.
-josh