Hello Nathan, the GAL functionality is implemented with MAPI. I'm able to create a new appointment and add attendee's using the same method. Can you go ahead and open a support ticket so we can get more information from you so we can fix this bug. Thanks!
MAPI does not currently run in the non-cached mode version. It's something we have planned to look into in the future.
Larry Duran
Software Developer
SmarterTools Inc.
www.smartertools.com