Even Greener Pastures

You probably know I love challenges (such as 30-Day Language Challenges) and I participate in a couple of them, therefore I  see myself as a “participant” but recently I came across the word “challenger” and it doesn’t sound right to me.

I have the impression “challenger” would be the person that proposes the challenge.
But how to name the participants?
Challengee? Is it common?
Challenge-taker? This suggestion doesn't seem popular.
Contestant doesn’t seem right, as there’s no competition involved.

Any suggestion?