There are all kinds of challenge teams here on SparkPeople, ranging from all kinds of interests and motivators. I know that there is a "Biggest Loser" type challenge that goes with each season. The summer challenge started a couple of weeks ago, and once the challenge starts, they don't take new members til the next season. But you can certainly sign up on their main page to be placed on a waiting list for the Fall challenge.
In the mean time, click on the Community Tab (above, in the orange row), and choose "Spark Teams".
When that page loads, there is a search box that you can put words into and then search - depending on your interests. I placed the word "Challenge" in the box, and came up with several teams. www.sparkpeople.com/myspark/groups_search_
Please give this a try. If you still need some help, please let me know.