Calendar sharing with open directory users

I have a server 10.8 running the Calendar service. One of my requirements is that certain users have access to the allowed calendars for those individuals.

We are serving calendar1, calendar2, etc... and I want to have user1 access ONLY to calendar1 or user10 only to calendar10.

Right now, any of our users can have access to ALL calendars as long as they have the server address and their user's credentials to open directory in our server.

I would like to share the calendars with specific users, but when I try only the email of that open directory user's shows up, instead of their name and the email is never sent.

Let me know,

Thanks.