I am looking for a totally free way to share outlook calenders without
sharepoint account or microsoft exchange server. If possible the
shared calender needs to update dynamically as entries are made. This
would be for a user group of 15 to 20 people and must be relatively
secure.

You're asking for a lot for a $5 answer. If you seriously have 15-20
users, you have options, but nothing short of Exchange Server will do
you justice.

From the ads, this link http://www.4team.biz/purchase_shareo.asp says
$30/per user, which is very reasonable.

If you're absolutely insisting on Free.. you can TRY some of the
solutions offered at http://www.slipstick.com/outlook/share.htm ...
You're asking for trouble.

Seriously, you might take a look at a complete groupware solution
(http://sourceforge.net/search/?type_of_search=soft&words=groupware or
http://sourceforge.net/search/?type_of_search=soft&words=sugarcrm) and
get off of Outlook altogether.

If you insist on staying with Outlook, well, you're welcome to try
OpenExchange... But you'll need a RedHat or Novell SUSE Linux box
(yes, free if you have a computer available) and some time and effort
to build and install it... Yes, it's not free if you want to buy it
all nice and prepackaged and with support...

If there was something for free that could do for Outlook what
Exchange does for Outlook, Microsoft would likely be out of the email
business.