As I passed comment before, if the coop felt having this as a private event would encourage people to join up to be a part of it then fair do.
But my thoughts are, it's a good event and will be one that encourages people to want to come.
Membership no matter how you dress it up is a sales process. You're trying to get new customers, and you cant convert customers as easy if they are not stood in front of you. And it would have been a great opportunity have the chance to convert visitors into members, with a happy well turned out event would be a whole lot easier than the cabin pre match.