Exchange 5.5 for remote users

Hi,

I have 12 user locations who plan to use Exchange 5.5 Server for mails.

If all locations have different NT domains & have WAN access (thru Router NAT, Proxy or Wingate).

How do I implement this feature for all locations ? Users must beable to put docs in the public exchange folder for others to share as well as schedule meetings etc.
Akash