The battles have officially begun as part of the Mega Melee Online Competition in Pokémon Sun and Moon. In the latest Online Competition, you may only use Pokémon that can Mega Evolve. Only one of the Pokémon from your team will enter battle, so pay close attention during the Team Preview and choose your Pokémon wisely! Duplicate Pokémon and items are not permitted. And every Pokémon will be set to Lv. 50 for the duration of the battles. Registered competitors can battle up to 15 times per day. The competition concludes on Sunday, September 24, 2017, at 23:59 UTC. If you missed the chance to register and battle in this competition, keep checking the Pokémon Global Link for more opportunities to battle.
Source: Pokémon Global Link