Yeah, I'm gonna back you up on that one. Sometimes assembling the group in session 0 is what's right for the story, and sometimes it really, really isn't. Think about how many movies literally have "Assembling the team" as almost their entire plot. The Avengers hangs two hours of non-stop action on "We need to put a party together." Every heist movie is basically required to have an "I'm putting a team together..." sequence.

Session 0 is where you lay out the expectations of the game, and your players think about either how their characters have already interacted, or how they will interact when they eventually meet. You give people an idea of what they're getting into, you pitch the tone and the style of the game, and you help people shape characters around that.

As an example a friend of mine always pitches his games by describing who they would be directed by. I remember vividly his "Lock, Stock and Two Smoking Halflings" game, a Warhammer Fantasy Roleplay If It Was Directed By Guy Ritchie experience. Just setting that sense of tone up front meant that we all knew to make characters who would fit the vibe. I played "Blackhand Seth, The Scummiest Elf You've Ever Met," one part Brad Pitt Pikey, one part Jack Sparrow, and I had a blast.

In my most recent campaign I'm running a Shadowrun game where the group would be assembled in session 1 by a down on his luck fixer. My pitch to the players was simple; make fuck-ups. I wanted characters who were at the end of their rope, lacking in options, either so green no one would trust them or so tainted by past failures that no one wanted them. The kind of people who would take a job from a fixer who had burned every other bridge. They rose to the assignment beautifully, and by four sessions in the group has already formed some absolutely fascinating relationship dynamics. A lot of that has been shaped by their first experiences together, figuring out how to work as a team, sometimes distrusting each other, and slowly discovering reasons to care about each other.