With outlook 2007 you have two options - you can send them the ical by email
or publish it to either office online or to a webdav (iis) server you have
control of. Right click on the folder you want to publish and choose send by
email or publish to internet. If the calendar doesn't need updated
frequently you can send it by email - otherwise publishing is better as it
updates hourly. If you publish it to a webdav server you'll need to give the
others permission and block anonymous access.