I dont know if there is on Discord. I haven't even found a way to talk to other people aside from the group im in. I would suggest looking on Steemit for other people doing food challenges or just on a diet. Then follow and comment on their posts. Get yourself known to them and then create the challenge for the people or talk to someone about the challenge in that category, saying what you are planning on doing and do it with them. It is about knowing the right people sometimes.