Ask HN: Math/programming problems solving in groups

Hey everyone! I am interested in improving myself as a developer by solving some Project Euler[1]/Matasano crypto[2]/hackerrank[3]/etc challenges. At the same time, I really need to socialize as much as I can. So I thought about starting a Meetup.com group for this. I am just not sure how to organize these events so they are as productive/fun as possible. What would be the best approach? Everyone solving the same problem doesn't sound like a good option just because for some people the problem might be difficult and for some - easy. Also, I don't want this to become a some sort of "who is the smartest" competition. Maybe you participated/organized an event like that somewhere? I would love to talk to someone who already tried organizing that kind of event(-s)!

[1] - https://projecteuler.net/ [2] - https://cryptopals.com/ [3] - https://www.hackerrank.com/