That needs to be something web-based then.
Things to research:
1. hosted Exchange (just google for it), that's MS Exchange via webmail hosted by some third party, not on your own server like bigger companies tend to do. You've got shared mail, shared contacts, shared calendars, shared folders.
2. Google services (Google calendar; gmail with address book)
3. Microsoft services (various services offered as Windows Live)
My recommendation would be #1, I think.
Need to solicit some feedback please. Our small office is growing and we need to be able to share calendars and contacts. Two hitches: 1) we have PC lovers (Outlook) and Mac huggers (Entourage and Address Book/iCal), and 2) some of the guys are off site. I need all of these guys to be able to share and sync their data.
I've done the google search thing, but am now looking for some real world feedback. Any suggestions?