Hi Shane,
This would give you the # of attendees (multiplied by 5)
{% OnlineMarketingContext.CurrentContact.ContactGroups.Where("ContactGroupName.contains(\"Attendees\")").Count*5 |(identity)GlobalAdministrator%}
However i don't see a way to dynamically set the "Score" in a rule currently. So you may need to create 5 separate rules with 5 points each.