Basically, I want to put a contact management directory online and have it updateable and searchable by any number of secure login users via the web. I'm sure this exists somewhere, but who knows which ones are good?

I'm hoping to find something pre-built that I can readily customize, adding fields and such to make it work.
-- access online from multiple users, who maintain and update a profile
-- fully searchable by various categories (e.g. language spoken, education)
-- customizable fields
-- easy to use (will be accessed by non-tech people)