Something seems wrong here:
1. Create a distribution list containing User A. Make sure that User A has Mail/iCal configured for Zimbra.
2. Have User B send a meeting invitation to the distribution list.
Expected result: User A gets the invitation in Mail, the event shows up in iCal with a dotted-line, colorless rectangle, and the user can accept it or decline.
Actual result: User A gets invitation in Mail, the event shows up in iCal as a colored rectangle which cannot be accepted/declined. However it shows up in Zimbra Web Client with Accept/Tentative/Decline.
Problem seems to be that iCal looks at the .ics file and sees only the distribution list address as an attendee. Conversely, it's not clear if ZWC is behaving properly when it "accepts" an invitation even though the user isn't listed as an attendee.
EDIT: Hm, RFC 2447 suggests that the email address of the individual attendee must appear in an ATTENDEE field in the iCalendar object. If so this suggests that distribution lists should not be used for sending meeting requests, since the mail client and iCalendar application used to send the meeting request have no way of knowing the email addresses of the people on the distribution list.