The problem with your post is that there are two different aspects to your problem, the server side - which is quite easily solved using OpenLDAP or any of the other LDAP servers out there - and the client side, which you have failed to specify at all.
What you appear to want is the shared address book / calendering functionality of a product such as Exchange or Lotus Notes, but without you specifying what the client would be nobody can tell you what would be the best.
If I were to write one from scratch, then I would use OpenLDAP, and choose my LDAP schema for address book and calendering (may have to make one up for calendering - I don't know if there's a standard one) and I would then develop my own standalone client written in Java to access the LDAP server to display the information retrieved.
Additionally I'd probably write a web front end so that people who didn't want to install a fat client would also be able to access it.
However, if you're talking about integration into existing client software, then nobody has a chance of answering your question until you tell us what the client software we're supposed to be integrating with is.
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