Exaggerated: The school-girl is a Funny Foreigner Martian, the nerd's trying to summon demons, the twit races cars on foot on a daily basis, and the beauty queen talks to animals. All the while, Brad's the only one who notices that they're off-kilter.
Everyone but Brad has genuine psychiatric disorders; the schoolgirl has dissociative identity disorder, the nerd behaves like a shell shocked Vietnam veteran (complete with references to being in 'nam, despite obviously being far too young for that), the musclebound twit speaks to the voices in his head, and the beauty queen is under the impression that the demons going around stabbing everyone "just need a hug". Brad only stays with them because he's in school to become a psychiatrist and thinks its good practice.
Zig Zagged: Brad is the Only Sane Man when the team is fighting crime, keeping his cool while his buddies panic. However, Laura (being the Token Female) is the Only Sane Woman when Ms. Fanservice walks past, sending all the guys, including Brad, into a frenzy. And when the group goes to see The Only Sane Men in concert, the baton is passed to Joe, as he's the only one in the gang who isn't hopelessly obsessed with the band.
Everyone is portrayed to be relatively normal. Or at least two people.
Alternatively: Everyone is completely crazy.
"We need some sane person in this show; otherwise, no normal person would relate."
Alternatively: "We need to throw in a sane guy to highlight and to react to the insanity of the rest of the cast."
"It's like I'm the only normal person around here!"
"Please. If I wasn't around, who'd point out how dumb you were acting?"
Invoked: Brad tries to find some insane friends. So he can laugh at their wacky antics and make snarky remarks about them.
Exploited: Brad uses his insane friends to make himself look better by comparison.
Defied: Brad finally gives up, lets loose all of his inhibitions and becomes just as bizarre as everybody else.
Discussed: "Brad, why on earth do you hang out with those weirdos?"
Conversed: "Is it me or is Brad the only normal person there?"
The antics of Brad's friends bring him to a breaking point, and thus drive him to insanity.
Alternatively: Brad begins to believe that they´re all retards and fuckheads, and starts to wonder if he should change group.
Alternatively: Brad mostly has began to isolate himself from the others, because he can´t stand them anymore.
Brad is actually an Evil Overlord, but literally no one else knows it.