Hi Eva,

this is not possible with Outlook, only with 3rd party tools.

have a look on this site :

Outlook sharing solutions:
http://www.publicshareware.com/public-share-outlook-description.html
http://www.slipstick.com/outlook/share.htm

Article about "sharing your Outlook information (by Eric Legault)"
(Only share/view calendars, not edit other calendars)
http://office.microsoft.com/en-us/outlook/HA011477571033.aspx

Office Marketplace Outlook messaging tools:
http://office.microsoft.com/en-us/marketplace/CE010719621033.aspx

Maybe it helps.

--
Oliver Vukovics
Share your Outlook PST files without Exchange: Public ShareFolder
Synchronisation for Outlook and Exchange: Public SyncTool
http://www.publicshareware.com