Serial entrepreneur Cal Henderson met his co-founder, Stewart Butterfield, the Silicon Valley way: By hacking into his email server and gaining access to company data.

"What he was doing was super interesting, and I wanted to get involved, but I was just a nobody working in London doing web stuff," Henderson recalled, speaking in conversation with Mat Honan, BuzzFeed's S.F. Bureau Chief, at the Collision Conference in New Orleans on Thursday.

At that time, back in the early 2000s, Butterfield was building out Game Neverending, an interactive, multi-player game system, through a company he'd founded in Vancouver, B.C. called Ludicorp.

"Obviously, what I did was break into the email server and join their internal mailing lists to find out what they were doing, and then use that as leverage to convince them to hire me," Henderson continued. It worked, to say the least. Together, Henderson and Butterfield went on to launch Flickr, the photo-sharing service that was acquired by Yahoo for roughly $25 million in 2005.

Today, they're best known as the operators of Slack, the workplace messaging service that recently raised $200 million for a massive $3.8 billion, "unicorn" valuation, and won the 2015 Inc. magazine 'Company of the Year' laurel. The business counts about 2.7 million daily active users, and 800,000 paying customers, including 80 percent of all Fortune 100 companies, according to Henderson.

The company takes revenue through a "freemium" model, meaning the core service is free, though clients pay extra for enhanced security, say, or to integrate more third-party apps.

Slack may be a darling of the technology industry--and a verb, à la Google, unto its own--but Henderson is adamant that scaling the company has been a major challenge.

Initially, Slack's customers were other California startups, which typically count fewer than 100 employees. Over time, and as the company continues to grow, it's needed to build out new systems to support up to hundreds of thousands of users operating on the same platform, which facilitates not only messaging, but also bots and other applications.

"If you do any kind of business and you use computers today, you use up to 50 different software products to get work done," Henderson explains. "Communication is where it makes sense to tie all of them into one place."

Serving the major players

As Slack has grown to attract larger companies, including hospitals, universities and newspapers, Henderson and his team have had to build out new features, such as more robust administrative controls, security and access controls, and HIPPA compliance tools for healthcare systems. Consumers are increasingly asking for message threading, which Henderson says is also in the works.

It's the "unsexy end of business software," he said. "Once you start to think about tens or hundreds of thousands of people, then the product design starts to become a bit different."

Slack's Twitter account (@SlackHQ) receives tens of thousands of recommendations each month, which are factored in--and responded to--by a human company representative. "People are unafraid to be critical in public, and it's a really rich channel of feedback," said Henderson.

Making diversity a corporate priority

Silicon Valley employers aren't known for being especially diverse. In fact, nearly two years after such companies were persuaded to publish their (disappointing) internal diversity statistics, the majority of tech workers are still white and male, according to the most recent available data.

Slack has reported a higher percentage of black engineers than most of these employers (7 percent), with 70 percent of the workforce identifying white. Overall, women make up 39 percent of the company.

For Henderson, those numbers reflect the fact that Slack made diversity a company priority from the very beginning -- despite being helmed by white executives.

"There are four co-founders of Slack, and we're all white dudes with beards," he said. "We focused on diversity very early on, when the company was sub 50 people. And I think that was a key thing for us, because if you're the first person of an underrepresented minority to join a company, and it's a company of 100 people, and none of them are like you, that's a lot more difficult."