The Tap and Spile has and will continue to welcome new faces. Judie Knight and Jim McDonald make a point of including any newcomers and encouraging them to sing. It has been discussed at length and of course if people meet together frequently over the years there will always be 'in' jokes but noone should feel that they can't join in.
As for keeping things a secret i think its more a case that folk say 'meet you in the ... for a session' rather than specifically setting out to exclude others. Session etiquette in my opinion is that you don't go in and try to hijack the event with something that doesn't fit but sometimes if someone says 'do you mind if ...' it will be greeted differently. I don't really think it is the responsibility of the festival organisers to publicise fringe events over which they have little control and have never felt that they try to ignore them either.