Hi Rich:
Somewhere you will need a server to make a good job of this.
The others are quite correct, you can share files and projects using the
Project Centre. However, you can only share calendars by saving them as web
pages then going to look at each other's calendars. Without a server, you
can't "share" your calendar so that others can see your free/busy time and
add appointments.
Entourage offers full calendar sharing only if you connect to an Exchange
Server. You could install a Microsoft Small Business Server somewhere on
your network to provide very rich functionality. The SBS server is an "out
of the box" solution for small business that provides an Exchange server,
File server, Web server, Messenger server, and SharePoint server that gives
you the ability to share almost everything except your wife!
An SBS box is small and cheap (basically, it's a PC, and unless you have a
lot of concurrent users it can be a "cheap" PC). Even though it's a
"server", it doesn't need powerful hardware until the number of users starts
to climb
See here:
http://www.microsoft.com/windowsserver2003/sbs/default.mspx
You can buy one as a plug'n'go solution for $1,365 here:
http://www.dell.com/content/products/features.aspx/rec_server_networksol?c=u
s&cs=04&l=en&s=bsd
You "could" run this up on an Intel Mac.
You might even be able to get it running in the background on one of your
desktop machines, using Parallels. If you want to try that, we'll put you
in touch with a specialist SBS MVP to help you get it going: it can be
tricky to install SBS if it is not the operating system in control of the
hardware.
Hope this helps