There are two ways:
1. Create a calendar group using the web client.
2. If you have groups, like listservs, in you LDAP server, you can invite all members of a particular group; when creating an event, go to Search... for invitees, find the group that you want, and invite it. If all members of that group have existing calendars, those calendars will be scheduled, if a member doesn't have a calendar, an email will be sent.
Hope this helps.