Can anyone help me set up a leaderboard for my quest game in which I can show the players how other users have performed?

At the moment you cannot do that. Hopefully in Quest 5.8 it will be possible by having your game communicate with another web server by PHP, so it will take some setting up. Quest 5.8 is currently in beta, and should be out around the end of the month. I hope.

Can I contribute to it in any way?

You can beta test! See the thread, I just released beta 5.