I'm not an expert because I haven't written posts for those challenges in a long time. But if memory serves me, anyone can join. You just need to follow the rules of each particular challenge which you can find on the dedicated posts.

Also, they have recently introduced a policy to benefit people who re-invest their rewards on Hive or in the game itself. That means people who withdraw most of their rewards outside of the ecosystem will be penalyzed but I don't know the specifics