You can’t join as a participant partway through, but you can join the next event (no date for that yet).
Voting is explained in the link at the top of the event main page.
The criteria for the next event haven’t been decided yet, but will probably be very similar to the first one.
All the information is available from the “Vote Their Clothes Off” section in the site’s main menu (the black bar stretching horizontally directly below the logo).
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