Adsense Exchange Teams are groups of people who are clicking on eachothers advertisements. It's not allowed from Google Adsense and it's risky as if they caught you, your account can get banned.
There are a lot of groups doing this though and I've been invited a lot of times to them, but never agreed to get in.
What do you personally think about these groups? Have you ever joined one or not? Tell us everything please!