ROCHESTER HILLS, Mich.—The folks at Life Time Fitness have taken to calling group workout classes “happy hour.” Dozens of members pack into a studio with other fitness geeks, and then proceed to sweat it out via dance routines, yoga sessions or kickboxing simulations.  But for many, the window of time leading up to happy hour is pure misery.  Latecomers typically face rows of mats already “reserved” by pushy patrons with towels, water bottles or jackets; laggards confront the dreaded red “CLASS IS FULL” sign. Managers at the hulking facility here in an affluent metro Detroit suburb have taken to stationing a supervisor at the door—a man some refer to as “the bouncer.”  So while people like Susan Brown, 59 years old, leave work early to make it to the gym in time for decompressing classes, they often arrive in a bundle of nerves.  “Do I have sense of urgency in getting here? That’s an understatement,” says Ms. Brown, who works in banking.  On a recent day, she and her 22-year-old daughter were among 40 people to line up in front of Studio 2 for the 7 p.m. “Dance Jam” taught by Alexis DeCapu, a local choreographer who has worked with the Detroit Pistons’ dance squad. Ms. DeCapu is one of a handful of local superstars teaching at Life Time’s Southeast Michigan network. While their popularity is an encouraging trend for the chain of fitness centers, the competition to work out with them can wreak havoc.  “It’s stressful,” says Christina Bishop. A mother of three and wife of a U.S. congressman, she places a high priority on getting to the classes as part of her weekly routine. She’ll go so far as to hop to another gym across town to follow an instructor to a different location.  Beth Vostokov regularly arrives to find a line outside a 50-person capacity studio for her 7:15 yin yoga class Tuesday evenings. “They just seem grateful that they’ve made it,” she says.  Gerri Avery, who works the front desk, has seen plenty of desperate pre-class moves. Some members try to bypass the long line that extends through the lobby and out the door. One common tactic involves anxious exercisers who sneak in without swiping their membership cards. Others who make it into the facility may do an unauthorized sprint down the corridors to ensure they get a spot in a class.  “They’ve got to mark out their territory,” says Ms. Avery, who functions much like an elementary-school hall monitor. Often, she has to yell out “no running in the hallway!”  Robert Ochoa, a Rochester Hills Life Time supervisor, has become the resident enforcer. He’s the guy who has to attach the “CLASS IS FULL” sign on the door when a studio is at capacity.  Members despise the sign and try to evade Mr. Ochoa’s gaze. He can’t let his guard down, or even break for a drink from the nearby water fountain. If he moves an inch, he knows, people will sneak in.  “It’s gotten to the point where they’ll cross through the Studio 1 door because they know I’m at the Studio 2 door. But I check the mirrors and catch them,” says Mr. Ochoa.  Others try to manage their time properly, even if it means a double workout. Anna Patrico, 20 years old, hates standing in the back of class, and has a specific spot in each classroom—such as left of stage in Dance Jam. “If I’m late, I won’t get a spot,” she says. To ensure she doesn’t have to suffer behind 75-plus swaying, jumping, patrons, she goes the extra mile—by attending the class held before Dance Jam. After the first one, she simply stays put in the studio. No waiting required.  Others are creative, too. Ms. Bishop has witnessed a committed few who will leave during a cycling class to run upstairs and mark a spot in line with a coat or water bottle for Cindy Lindner’s cardio-kickboxing class—and then return to finish cycling.  “Sometimes the first five spaces in line are towels and articles of clothing,” Ms. Bishop says. She believes she is craftier, having formed alliances with other members to save a space in Ms. DeCapu’s class.  She has seen strangers exchange phone numbers while waiting in line. They aren’t interested in dates—the once popular ritual at gyms. Rather, they’re interested in forming a backup network of contacts in case they are ever running late.  Life Time recently rolled out a smartphone app, on which members can follow classes and instructors of their choice. Mr. Ochoa says the gym has already noted people skipping class when alerted a favored teacher is being replaced with a sub.  It will be hard to shake the love for star instructors like Ms. Lindner, who boast their own student fan clubs. Devotees bring her health shakes and arrive 75 minutes before her 8:15 a.m. FIT class begins on Saturdays. Early birds take the initiative to set up the studio. And of course, they grab the primo spots for the session.  Ms. Lindner, who also works as a technology director in the local school system, has learned the art of leading a flock. She keeps a notebook in which she writes down things about regular attendees—where they stand in the classroom, what’s going on in their lives.  “I’ve got nothing without them,” she says.  Write to Anne Steele at anne.steele@wsj.com and John D. Stoll at john.stoll@wsj.com
Some yoga and other fitness classes have grown so popular that fans are going to frantic lengths as they jockey for a space to de-stress.