Shared Contact List and Calendar

Now that I have rid myself of our “Microsoft Mail” nightmare and moved to a Linux Mail server, I ran into some difficulty.
I have created a couple of Contact Lists, exported them, and directed our clients to them. Problem I ran into is that only one client can connect to a Contact List at any one time. So, I placed a copy of the origional Contact List in their Home Directories and they can access it there. Everything is GREAT, except that if I change the list, I have to push the list to the users.
There HAS to be a way to have a Shared Contact List WITHOUT having to rely on Bill Gates (Exchange Server) for it. PLEASE tell me how to do this!!!
erodriguez@vycera.com