There used to be a way to do this without exposing everything else like you would if you were sharing Contacts with someone else via iCloud.. the method use to be MobileMe before it was replaced by iCloud. However this change discontinued sharing via subscription. Bummer.
If you were to share contacts so that both sides have read/write capabilities, the easiest way to go about doing this is by adding the iCloud account to the other computer that you wanted to share with. You would then have to uncheck all other shared items though.. like Mail, Calendar etc. This is great if you sharing with someone you trust but what if you wanted to share contacts without exposing other data assoiciated with your iCloud account?
I read from a thread that a service called Fruux might be the answer. I have not tested this out yet to see if it works the way it was explained. Just from visiting the pricing page I can see that a free account is limited to sharing up to 2 devices and 2 shares. Anything more would require a subscription fee. Good to know that there might be a way should I ever feel the need to explore further.