Why wait to create a challenge? Why not create one of your own for others to participate in?
I had joined this group, because I'm working on being more frugal, especially since I'm seeing the light at the end of the tunnel with getting rid of my credit card debt as well as reaching my goal weight-wise, and just looking for tips, not challenges. Everyone has different reasons for joining groups.
As far as seeking sources of motivation, that should be coming from within anyways. Inspiration can always reinvigorate that motivation, but the source really needs to be internal for this process to truly be effective and permanent.
Just my two cents. :)
Camp Good Grief 5k 2012 - 31:50
Gobble Wobble 5k 2012 - 30:00
Turkey Trot 4 Mile Race 2012 - 38:38
Ugly Sweater 5k Fun Run 2012 - 27:19
Move-It Memphis 10K 2013 - 55:36
Germantown Half Marathon - 2:08:53
FedEx St. Jude Classic Fairway 5k - 4/13/2013
Pittsburgh Half-Marathon - 5/5/2013