The largest — and maybe the loudest — fan community in baseball started with a frustrated T-shirt.

Near the end of the 2009 Major League Baseball season, Darren Meenan — who was bouncing around from job to job — created a T-shirt to wear to a New York Mets game as a way to communicate with other die-hard fans who had "survived" the Mets' disappointing string of seasons.

"I was at almost every game, and I decided to make myself a T-shirt that said 'I Survived' to show that I was still there to be a fan of the Mets, even when they were losing," said Meenan, who created the T-shirt in his parents' basement. "The fan base in 2010 got the word out, so I decided to do it as a business. I created a website and got started on social media."

Meenan, the founder, has turned two lifelong passions — rooting for the Mets and printing T-shirts — into The 7 Line, one of the most successful fan-led companies in professional sports, with an army of New York Mets fans to support it. And it took only a few extra years for the Mets success on the field to catch up to Meenan's success in the Citi Field stands.

Today, The 7 Line is a fully-licensed MLB apparel supplier run out of a warehouse in New Hyde Park, New York. The company has a handful of employees, with only one other person working full-time with Meenan and "a couple of part-timers," he said.

As of 2014, a kiosk in the center at Citi Field stocks the one-of-a-kind gear that fans can pick up during games. And despite the intensity of the postseason, Meenan continues to believe in keeping the grassroots company small and doing what they can with their current, often overwhelming demand.

"My plan from the start was to not have a plan. I let the fans decide how big we want to get and grow," Meenan said.

He declined to disclose any financial information to measure his company's success.

Yet things have not always been amicable between the Mets and The 7 Line, which was named for the New York City subway line that takes fans to the Mets' Citi Field. Meenan was kicked out of a game in 2011 for exhibiting a sign that stated "Don't Trade Reyes" (a reference to then-Mets shortstop Jose Reyes, who went on to win the National League batting title that year before signing as a free agent with the Miami Marlins). The sign, which displayed the company's URL at the bottom, ended up being broadcast on television.

"The intent was never to anger the team, as the last thing I wanted to do was get kicked out of the ballpark," Meenan said. "Looking back on it now, it was wrong, because people pay a lot of money to get a license to display their website in the stadium."

Any negative publicity from the stunt has been quickly forgotten. Closely behind the establishment of The7Line.com came The 7 Line Army, a group of loyal fans who organize outings and sit together to show their fervor for the Mets.

"We'll sell all 859 seats in center field at Citi in a minute for a regular home game," Meenan said.

As the largest group of traveling supporters in baseball, they don't limit themselves to Mets home games. The Army travels around the nation to several games each year at opposing teams' stadiums. This past season saw their biggest away-game turnout when a group of 1,100 traveled to Pittsburgh. Two trips—one to Baltimore and the other to Colorado—drew more than 700 fans each despite being only three days apart.

"You'll see other teams' fans sitting together, but nothing like this," said Helen Lazos, a Mets fan who has attended games for more than 50 years. "I remember sitting in an empty row by myself with my husband for years before this. The 7 Line brought a lot of people together."

And the third annual "Bronx Invasion" brought more than 1,300 Army members crashing in on Yankee Stadium for an interleague series between the Mets and Yankees earlier this season.

"Nothing against the Bleacher Creatures, but after the roll call in the first inning, they're done and they don't leave Yankee Stadium," Meenan said. "We're the biggest group in baseball, hands down!"

Meenan is surprised that other teams in the MLB lack similar groups but has stopped giving out pointers to other fan bases about how The 7 Line built its success. Several groups have blatantly attempted to rip off its business model, but none have had any long-term success, he said.

The 7 Line's thundering support has been noticed by a very important group — the Mets team itself.

"In Baltimore, [outfielder Curtis] Granderson gave us a shout-out: 'The 7 Line Army is here!' And [pitcher Jacob] deGrom in the post-game interview said it felt like a home game. During that game, I had to look around and realize that we weren't at Citi Field," Meenan said.

The Mets trip to the 2015 World Series has given this long-suffering group of fans a lot to cheer about (even though, per Elias Sports Bureau, home teams that have won the first two games of a best-of-seven postseason series have gone on to win the series 80.8 percent of the time — though one of the few teams to dig out of a World Series 0-2 start was the 1986 Mets).

Meenan knows that it is easy to cheer for the team now, during a largely successful season, but he is grateful that his company's beginnings came during hard times. "It's good for the sport, and it makes people more passionate and brings the fire back, win or lose," he said.

Correction:
This story has been updated to reflect the fact that New York Mets player Curtis Granderson recognized The 7 Line Army at a game in Baltimore, and the Yankee Stadium visit was the third annual "Bronx Invasion."

—By Michael Sheetz, special to CNBC.com