Acute Softwares Network Diary - 5 user pack Description:

Allows small businesses and companies to share appointments from a central server. Each user on the LAN runs the diary Client, which keeps their own appointments, file usage and PC usage on their own PC. To make appointments public they simply tick a box and it is automatically copied to the server.

This is a 5 user License, (comes with the server and a license for 5 clients)
Upgrades are free for 2 years and full email support is included.