FIRST Robotics. Every team is run a little bit differently, but every team I've come across is always looking for more mentors. https://www.firstinspires.org/robotics/frc

A year ago I got involved with my local robotics team and began mentoring them as a way to give back. It's been very fulfilling, especially since the team has shown significant growth since then (3 students with 1 mentor => 12+ students with ~6 mentors).