I'm currently using GW 6.5 and can't recreate the problem, but on earlier versions of GW I've had that message appear many times because there was nothing to process.
Try having one of those 6 individuals send an appointment then try to run the rule. If you can't ask them to create an appointment, then add a 7th user to the list and have that person send you an appointment. If the rule doesn't error out, you know it works.
[tt]-John[/tt]
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