I agree with the idea of an icon for a no show - but only if there is absolutely no communication in the days immediately after the trip. eg: A car breakdown coupled with poor internet reception could cause a no show without notification, but I would expect an explanation within a few days.
And after a few icons then automatically bumped down the list when others join, .
I don't ageree with removing people who don't check in. Not everyone has internet access 24x7 and some people may book a trip to coinside with another trip and may be out of internet reception for the week prior to the 4wdtrip. eg: On a High Country trip with another group and meeting a 4wdtrip group in the High Country. Although with enough foresight this scenario could be mentioned when booking in.