With joining roleplays, I have no idea, I don't do roleplays. For helping people with Niche...I'm not quite sure what there is I can say? You just help people out if they need it. And finally, for making challenges, you should look at the ones others have posted below. Then you can make a new topic there and make up a challenge and put it there.
https://strayfawnstudio.com/community/index.php?/forum/78-challenges/
Let me know if you need any more help! ^^