Here's a response to a very similar question I gave to a 3rd party company we work with recently:
Unfortunately it seems that you need Exchange to do this with outlook. There's an article here about some workarounds but none are 100% reliable:
Depending on how big your office is and whether there are plans to grow it might be worth looking at buying Exchange or something like it for shared calendars, email etc. There are almost certainly free tools that do similar things out there but it is nice to stick with what you know. A quick search on google for "outlook calendar sharing without exchange" revealed the following (and lots of other solutions) which your IT provider might want to investigate (I haven't tried it but it sounds like just the thing and appears to have a free trial): click here
It looks like it costs approximately $40 per user and sounds like it would allow you to share your calendars and stick with what you have - you'd just need one PC/Server to act as the 'server' for it.
(additional comments below:)
I guess you want to use Outlook? If not there are various shareware calendar/web based tools out there. Google have a calendar for example.