It appears that there are now two ways to enter Splatfest Battles: Solo & Team. Solo mode works like Splatfest in the original Splatoon game; you’ll automatically match with three other players and battle against groups of four random players from the opposing team.
Team mode requires four players (must be friends with someone from the same team who is in the Lobby) to join a private lobby before being matched up against another team of four.
The final result will be determined by popularity, Solo Win percentage, and Team Win percentage.
— Squid Research Lab
Source: Splatoon’s Tumblr.
…Wanna play? Buy a Switch.
And if you’ve already got yours, click on Splatoon 2 for everything you need to know about the game. 😀