Good question. . . I have looked and it doesn't appear to be written down anywhere. You just kinda make it up as you go along. Ask for joining intructions and your given the usual reply then you request something more specific like, 'to join at low level from the south'. If you sound like you know what you're talking about. . then your in!
You'd get a better response if you posted on the 'Rotorheads'.