People will join depending on whether they are interested, and of course whether they are available. 1800Z is a good time for most people, so try making your Original Post look more appealing. A good one to go off are the Events ran by the Tailwinds Flying Club, which always look great. Here’s one. Also, as this event is so far away, people may wait a little while before they reserve a spot so that they know if they will have irl commitments on the day of the event.