In an interview over four months ago, Iksar talked about Hearthstone Battlegrounds and a potential mode for eight-player lobbies.
Dean Ayala: […] There’s just a bunch of features that we want to do first, like for instance getting 8 player lobbies, so you can queue up with your friends, but also for organizers outside of Blizzard to also make their own tournaments and broadcast those for people to watch. So that’s really the first feature. […]
Now a user found the first proof for the new way to play Hearthstone Battlegrounds!
But not only will you have the option to play friendly Battlegrounds in a group of eight, but you can also play regular Battlegrounds with up to 4 players in a group.
Note: Information based on Data, not officially confirmed