It is opt-in, but typically both opt-in and out-out in these sorts of contexts refer to what happens in the absence of user input. In that context opt-in has a

It is opt-in, but typically both opt-in and out-out in these sorts of contexts refer to what happens in the absence of user input. In that context opt-in has a bit of an issue with people even knowing it exists, not to mention caring enough to go find the option to opt-in, and then doing so. This route will, for better or worse, produce a very self-selected group. The proposal above is a way to maintain the benefits of opt-in (namely, consent) while trying to improve the quality of the group.