Perhaps to their surprise, it seems that Sariel has not taken one of the griffons. Therefore, each adventurer has their own griffon, except for Yalko and Aghanim, who share one. One of the two will command the griffon, while one will need to be a passenger.
It seems that the temple and lady Kava have arranged for the stablekeeper's payment, as he does not request any from the adventurers and lets them know that it was on behalf of the Order of Blue Fire. With that, the now-party-of-six takes to the skies on their new mounts.
The griffons seems quite well-adapted to flight, and follow commands faithfully and intuitively. Even those not well-trained in riding are able to keep their griffons on course with ease. They are a little smelly up close, but that can't really be helped. In any case, your best information places the largest earthmote to the southeast, so that is perhaps a good place to start looking.