There is a good chance that some are paid performers. If they are, it is likely that one Adam Swart put them there.
Los Angeles-based entrepreneur Swart has pioneered the 'supporter-for-hire' business.
"The company started as a way for up-and-coming celebrities to get more attention," explains Swart, a UCLA political science graduate. "If you are surrounded with bodyguards and paparazzi you are taken more seriously."
"Then we started fielding political requests - to provide crowds for world leaders and US political candidates at federal, state, and local level, or to conduct rallies and protests in support or opposition to an issue."
The company repertoire has now expanded to cover product launches, PR stunts, and social justice movements. One popular side-line is using crowds to apply pressure to one side - or even both - of a corporate litigation dispute.
Swart claims the business has "more than doubled" in size each year, and can now call on tens of thousands of performers to cover several events each day, in dozens of cities across the US
The next step is going global.
"We get a lot of requests to do international events... but we have to make sure we are operating within all applicable laws," says Swart. "Many countries don't have the same protection of assembly."
The CEO hopes to begin with the UK, Dubai and India within 18 months, testing the waters with uncontroversial celebrity events.
Crowds you can control
The return on investment for clients is variable, but Swart is confident his services offer value.
"When a clients spends $10,000 on a protest and wins a $20 million settlement, that's a clear return on investment," says the entrepreneur.
"Other situations are harder to decipher -- it may be more 'can we shift the media narrative?' 'Did we get more press coverage or photo opportunities?' There are many ways to judge our services and their effectiveness - some quantitative, other qualitative."
The role of crowd members is carefully calibrated. Recruits are generally actors, who are expected be enthusiastic, but not so zealous as to risk arrest. Critically, they must look and sound authentic.
"They almost always hold signs and chant, and sometimes talk to media on behalf of the event," says Swart.
Employees also sign non-disclosure forms that protect the client's anonymity, to avoid the embarrassment of their paying for support coming to light.
In rare cases where the practice has been exposed, such as with Donald Trump's hiring of supporters
, it has been heavily criticized.
But Swart is comfortable with the ethics of his business:
"We didn't invent this," the entrepreneur says. "People are just looking for ways to spread a message in an unconventional, interesting and effective way. Before this you could spend $100,000 to put an ad in the Washington Post. Now you can spend a tenth of that to get right in front of people."
Tip of the iceberg
The business model is spreading. Ukrainian company Easycrowd predates Swart's company, while Crowds for Rent and Rent a Crowd were recently launched in the US and UK respectively.
These novel companies represent only the most visible feature of a sprawling landscape of manufactured support.
"There are similar types of consulting firms that provide a vast menu of 'grassroots' services," says Dr. Edward Walker, Professor of Sociology at UCLA and author of Grassroots for Hire
"These include mass letter-writing or emailing, ghostwriting blog posts, locating data on likely sympathizers for your cause... even creating whole new 'front' or 'third party' organizations to serve as a mouthpiece for the campaign's funder."
"Considering all of the PR and lobbying firms that do what I call 'grassroots for hire'... there are hundreds of such firms across the country. By my estimate, around 40% of the Fortune 500 appears on the client list of at least one such firm."
Walker sees technology as a driving influence of the trend, citing Uber's use of its network
to pressure New York Mayor Bill De Blasio -- with a 'De Blasio button' on its app -- as an example of commercial companies' increasing ability to mobilize users to campaign on their behalf.
Another driver of the business is the growing value of crowds in the age of mass media.
"The appearance of grassroots support is critical for gaining attention on social media," says Walker. "It is critical to have an enthusiast community that can support your cause and act as ambassadors for it."
The Professor adds that companies risk reputation damage if their use of paid supporters is unmasked, but the practice seems set to grow, particularly in the arena of politics.
"The rise of campaign finance makes it possible, (and) the massive expansion of the PR industry," says Lisa Graves of monitoring group PR Watch. "As more big money is injected into campaigns... these efforts to deceive the public with fake crowds will continue to expand."
For entrepreneurs such as Swart, the current surge in demand for hired crowds offers boundless opportunity.
But as the industry grows, so too will the challenge of remaining hidden.