Separate groups in calendar server?

I have a situation where two companies are sharing one server. While everything is fine regarding file sharing, I'm having an issue that all calendar users can see each other. For example, if I create a new calendar and want to share it with someone else, I get autofill suggestions of all available user accounts on this server. I would like to be able to assign users to groups so that only users in the same groups are visible and others ignored. I'm guessing it's a long shot but thought I'd ask. Thank you.