Are you using an Exchange server as well?

Are you using an Exchange server?  If you are using Exchange you create a new
calendar item in Public Folders.  This is the best way to do this.  Right
click on the public folder that you want to create the calendar in and then
select New Folder.  Create the new folder as a calendar item, you can then go
into the properties > permissions to determine who can do what in that
calendar.

If you do not have Exhchange, you have to share an individual's calendar.  
You do this by opening the calendar and then choosing Share My Calendar from
the navigation menu on the right or from Folder View right click on the
Calendar and then click Sharing.  You can then choose users from your GAL to
share that calendar with and you can assign them specific permissions.

I have assumed that you are in a windows domain and that you just want to
share the calendar with internal users.  

If you need more help just post a reply in this forum.

If you don´t have an Exchange:

On this site is an article on various ways to share Outlook data:
http://office.microsoft.com/en-us/assistance/HA011477571033.aspx

You will also find a list of third-party sharing tools:
http://www.mapistore.com/Outlook/Groupware
http://www.slipstick.com/outlook/share.htm