Calander Software and networkable

Ok what I have is a secretary that has 3-4 people that she is trying to keep track of, and would like a calander system that she can do electronically. And she would like to to be avalible to the upper brass @ any of our 19 locations that they might be at. We do not have email in house so no exchange server or anything like that, that we can use.

Any suggestions?
Please?
Thanks!