Login to the webmail system
Go to the settings tab (gears on the left hand side)
Expand the section on sharing
Click on shared resources
Select my calendar and click next
Select the users or groups with which you wish to share your calendar (note this is limited to your domain, so only users with the same @<company>.com addresses). You may hold down the ctrl key to make multiple selections.
Choose the access level you wish to give those users in the drop down box below.
This may be done multiple times to give different access levels to different groups and/or users.