TORONTO -- With Choir! Choir! Choir! rehearsals almost ready to begin, the excitement among the pop collective's members is rising by the minute.

About 150 people have squeezed into the back room of Clinton's Tavern, a bar in downtown Toronto, on this chilly night in late November. They're about to sing Elton John's "Tiny Dancer," and many are whispering the 1970s classic under their breath, reading a lyric sheet they were handed with their $5 admission at the door.

Judging by the energy pulsing throughout the crowd, tonight's performance could be huge -- but it's not going to be easy. In under three hours, they'll learn how to perform John's six-minute song in harmony, under the guidance of Choir! Choir! Choir! founders Nobu Adilman and Daveed Goldman.

Once it's ready, the amateur singers will launch into a final performance uploaded to YouTube. It might even become their latest viral video.

The whole experience would've been unimaginable eight years ago when the creators of Choir! Choir! Choir! began drop-in sessions at a Toronto real estate office.

But over the past few years their simple idea has grown into a cultural movement.

Whether it was tributes to the late David Bowie, Prince and Gord Downie, or the recent heart-pumping collaboration with Rick Astley on "Never Gonna Give You Up," each video showcases a sea of anonymous strangers living out their jukebox glory.

Yet there's a mystery as to why Choir! Choir! Choir! keeps getting bigger.

"I love that there's so much mystique around what is, at its core, probably one of the most simple things," says Adilman, 46.

"People have been doing this for hundreds of years. We've just packaged it up in a different way."

This year, the choir defied critics who dismissed it as a cheeseball fad by drawing some of their biggest names yet, including musician David Byrne.

The creators have more ambitious plans, including a tour where they'll use ABBA songs to recruit local choirs for shows in Vancouver, Calgary, Winnipeg, Ottawa and other cities. They're fully invested in the project, with corporate gigs helping to pay the bills.

None of this seemed possible in 2008 when Adilman, a one-time CBC host, was stewing in an artistic funk at his Toronto home.

He'd just moved back from Halifax, a place where the arts community had welcomed him with open arms, and was having trouble finding his place in Toronto's creative world.

With spare time on his hands, he agreed to host a small choir for a friend's birthday party. It was there he became acquainted with Goldman, who joined him in leading the group in a cover of Pilot's 1974 pop hit "Magic." The night was a success, and Adilman and Goldman became fast friends.

With Goldman working days at a Toronto brunch hotspot, he wasn't in any rush to launch a singing career, but he couldn't stop thinking about that night. Every so often the two men would grab a bite and talk about making it happen again.

Two years later, they created a Facebook event titled "Choir! Choir! Choir!" -- a filler title they deemed a beckoning call to prospective singers.

Word got around fast.

On the first night in February 2011, the duo gathered 25 people inside the Bosley Real Estate office. A photographer at a Toronto weekly snapped photos of the event, billing it a "rogue choir practice," which drew more interest.

"We had so much fun that we told our friends we'd do it again sometime -- and everyone was like, 'Tomorrow, let's do it tomorrow,"' Adilman remembers.

"And from that moment it was every Tuesday, every single week."

Eventually, a second weekly session was added.

Even the most obscure songs were drawing a good 60 people, Goldman says. Elliott Smith's "Needle in the Hay" and Counting Crows' "The Ballad of El Goodo" might've been strange choices, but they were meaningful to the duo, so they sang them anyway.

"We could've been doing 'Ring Around the Rosie,' and they would've come," Goldman, 42, adds.

Letters began to pile up from choir members, another reassurance they were doing something right.

Adilman remembers the first one he received, written by a woman whose husband died from an illness. She described grieving mostly in isolation with her son before deciding to attend one of the sessions in hopes of reconnecting with a community.

"What's happening in the room is so positive it has this chain reaction," Adilman says.

"It can find its way into various aspects of your life, if you're willing to let it in."

Other artists started to hear about Choir! Choir! Choir!, building momentum. Pop sisters Tegan and Sara asked them to perform at the 2013 Polaris Music Prize and later the Juno Awards.

Posthumous tributes such as David Bowie's "Space Oddity" at Carnegie Hall have given the choir a reputation in some circles as the pallbearers of pop, a label the founders staunchly reject. While they've paid homage to musicians lost, it's part of a larger project spanning many corners of popular music, Adilman insists.

One of those tributes is happening tonight with "Tiny Dancer," although it's not going smoothly.

"Blue jean baby, L.A. lady," Goldman sings as he strums the guitar. He pauses to listen to the choir sing it back, and winces as they linger too long on some of the words.

"L.A. lady," he sings with pronounced finish. "Lady. Lady."

When the choir breaks around the 90-minute mark, Goldman says they're making progress, but he's not completely satisfied. Tonight probably won't be his favourite performance, but it'll have to do.

The choir leader's perfectionist tendencies are familiar to the singers.

This isn't a spectator event, the group is often reminded, sometimes with a flourish of F-bombs from Goldman.

Longtime participant Jane Mundell has witnessed her share of friendly clashes between the choir and their masters. She's been coming for three years, and remembers one night when Goldman pushed the low harmony section to the brink. They turned their backs to him in a sign of protest.

"It was as if they were on strike for how they were being treated," the Toronto teacher says with a laugh.

"Some came back with shirts the next week that said, 'We the lows,' as a show of solidarity."

Those conflicts are all in good fun, she says. They also weed out the softies.

Every so often a new participant walks in having watched the heart-warming YouTube videos, and stomps out shocked by their potty mouth choir lessons.

Goldman says he "almost intentionally" leans into vulgarities on stage to weed out the "old ladies who think they're coming to Kumbaya Fest, 1963."

"I will not be the leader of a church choir," he says. "I'd rather offend everyone in the room."

Another challenge with being so popular is choosing songs for the crowd. The duo is known to journey down the rabbit hole of debates over which classics pass muster.

Recently, they questioned whether the Backstreet Boys' "Everybody (Backstreet's Back)," could work as a harmony. It did.

However, the Eagles' "Take It Easy" was an absolute disaster.

Others songs seem unattainable, like Queen's "Don't Stop Me Now."

With "Tiny Dancer," once the kinks are worked out, tonight's performance looks like a winner. Goldman seems pleased with the results and Adilman readies the crowd to perform for the camera.

A week later, the clip is on YouTube for the world to see. Some people will share it with friends, spreading the Choir! Choir! Choir! gospel to prospective newcomers.

And before long, another generation of its members will be born.