How do I give some users access to ONLY a subset of contacts?

How do I give some users access to ONLY a subset of contacts?

We have a long list of contacts in Zoho CRM.  Most users have access to them all, but we want to give a few users access to a subset of them only.  How do we do this?

Background: each contact has been referred to one of a number of our partner organisations.  We want our staff to see all contacts but we also want to give user access to people from the partner organisations - but we need to restrict their access to only those contacts who meet a certain criteria (ie referred_to = "ABC company").

Actually, the ideal would be to restrict users who belong to ABC Company to only seeing contacts referred to ABC Company (that is, something like User.Company = Contact.referred_to).