In the opening moments of a 2000 episode of "Friends," Monica is trying to convince the gang that she doesn't have the flu when there's a knock on the apartment door. The six characters react as if Central Perk just ran out of coffee. Who dares to intrude on their exclusive club? The moment probably won't make the highlight reel during "Friends: The Reunion," available Thursday on HBO Max, but it's one of the most telling bits in the sitcom's 10-year run. The show may be one of the most beloved in TV history, inspiring everything from haircuts to bad pickup lines. But the vast majority of viewers were never really welcomed to be part of its world.