It's somewhat common to see XN9 on UA flights, so if the person whose account is being used for the tickets is a Premier member or holds a qualifying United-branded Chase credit card, it's not impossible that everyone would be able to get onto the same flight on a Saver Economy award (XN is the booking class for those passengers). Adding a stop in KEF may be quite challenging, though, as I don't think Aer Lingus serves Iceland; you'd end up having to fly more than twice as far by adding a connection on the European mainland, and there's no expanded award availability with partner airlines, so you'd likely be looking at splitting people up amongst several flights.