If it looks like it won't sell out to members, the club will put tickets on general sale- i.e. anyone can buy one.
If it sells out to members, the ticket exchange will open, i.e.anyone can buy one.
So anyone will be able to buy one, at some point. Don't panic.
I'm sure the club will put tickets on general sale of there is any danger of not selling out.
So you will either be able to buy through general sale, or just wait for the exchange tickets to appear.
#dontpanic