When a new BringIT Challenge is available you will see it on the "home" page as a notice in the "Action Area" of the site where you do your daily check in. Or you can find it listed on the News section. Either way, once you get to the Challenge page you will see the rules and can Accept the challenge using the "+" navigation element.
BringIT challenges need to be accepted by the member to participate. Your data for the challenge steps, activity minutes etc... will be visible to other challenge participants for the duration of the challenge.