The therapy may actually be exhibitionist in nature, but on the flip side of the coin, there are thousands of voyeur-type members of these communities who log in to read the latest updates of secrets that are posted.
Many people view the more exhibitionist approach as a way to signal a lack of inhibitions, but there are just as many people reading your ad who will interpret this as an invitation for a one-night roll in the hay.
An open bikini, to those not in the know, has enough of a peek-through look to satisfy your exhibitionist side, but it has enough coverage that it doesn't look like you are going nude.
Unless you're a total exhibitionist, chances are you want to avoid a "bikini oops" as much as possible.
Dipping by yourself will only give your companions the impression that you're an exhibitionist.