To enter a Challenge, click on the purple flag with a gem in your dungeon; the flag will also show your current place in the challenge:
If the Challenge has not yet started, you will see the following icon:
Click on it to see your results in the previous challenge:
However, a player's level will only be taken into account at the start of the Challenge when forming groups. For example, if a level 10 player reaches level 11 during the Challenge, they will remain in the first group.
The reward for taking part in the Challenge depends on the level of the group. The higher the level, the greater the reward.
You can see your current ranking and reward on the Challenge screen.
Players are ranked on the leaderboard. You can view the leaderboard by scrolling down in the Challenge window.