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I am looking for such a function as well.
In
Extcal there is such a nice function to participate or cancel. You can see exactly how many people have signed up for the event.
It is awesome with ExtCal, but I am using PiCal (since it has better repeat date management).
So if anybody can extract the code from ExtCal and make it into a block, it would be great.
I'll put in $20.- as well