There are a few different challenges going on, like USMC said. There's the T-ransformation, the 405 Deadlift EMOM, the general Lifting Comp, plus the bi-annual Photo/Video Check-ins (not so much challenges, but still something everyone can participate in).
All of those things started organically, by members, and developed on their own. I know that TN HQ does keep an eye on them and certain posts do sometimes get featured on their social pages (but not plugging the member's own separate social pages, because that's a whole other thing).
The thing with doing challenges on any kind of regular basis is that, A - Obviously you need enough people to participate. Not talk about participating, but actually following through and doing it.
B - You need challenges that don't interfere with their regular training too much, so people can just jump in and do it instead of needing to specifically train for whatever.
And C - You need watch the line between new/interesting and too regular/disinteresting. The Photo/Video check-in, for example, has been going on twice a year for a few years and there's been steadily less people posting in it, unfortunately.
So the idea in general might be something that gets more organized and "officialized" down the road but, for now, any stuff that comes together is best done by the gang here itself. That's not to say HQ might not be able to throw some kind of thank you (tangible or otherwise) to some of the folks who do step up and take part in forum activities.