Who were Brett and his crew in Pulp Fiction?
What did they have to do with Marcellus Wallace and his case?
Just like the briefcase, the purpose of Brett and the other guys isn't really intended to be known.
The trope that this matter represents is a Noodle Incident, which is when something from the past is referred to but never explicitly talked about, but we may still see the outcome of that something from the past.
We don't really need to know who Brett is or what he and the other guys were doing with the briefcase, how they got it, whether they were supposed to have it at some point, or what they planned to do with it. The backstory of Brett and his guys, along with the briefcase, are nothing more than MacGuffins, or plots/items that exist for the sole purpose of giving the story a reason to exist.
Like MattD said, Brett and his friends are only there as a plot device. They apparently double-crossed Marcellus in some way, but their purpose for being in the film is to show how brutally and remorselessly Jules and Vincent execute some dumb college-age kids who were in way over their heads. It's essentially a character moment for Jules and Vincent.
He and his crew stole from Marcelus. Whether it was from holding out on paying him as part of a deal or if Brett directly ripped him off is unknown.
Say what again. I double dare you.