The lobby of a Dynamax Battle now offers two paths. Battle matches you automatically with other players. Private Group asks for a group code and puts you in an existing group, which is handy for running battles with your friends.
The change arrives with Pokémon GO Wild Area 2026, held in Sendai, in Mexico City and worldwide in November, and it also applies to later events.
The full event is covered in our post on Pokémon GO Wild Area 2026, and the source is the official announcement.