When you become a member of a club, your details are added to the AWA list on their web site (including AUS sail number). - There is a lag for many reason.
The day before an event the organiser only needs to print out the list and see if your name is on it.
If your name is not on the list (for what ever reason) and you have joined a club, then it is only a matter of telling the organiser that is the case.
The responsibility is actually on the participant, as the organiser can't get in trouble if you have not told the truth - in the advent of a public accident.
If the participant chooses not to tell the truth, then good luck to you...
A bit Sad though

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This is done on purpose, as it take the pressure off the organisers and puts it back on the individual, which is only fair....
Regards.