A business customer can use Slack’s new feature to set up collaborative channels with external partners, such as advertising or public relations agencies. In the past, a team at company A could admit guests from company B, but that was done on an ad hoc basis.

“No company works as an island,” April Underwood, Slack’s vice president of product, told Fortune. “They all have vendors, partners, and customers all of which can benefit from working in channels.”

The new shared channels have been tested by about 40 customers for the past few months and now customers with paid Slack Standard or Plus accounts can sign up.

“Shared channels are the most important thing we’ve launched since Slack itself,” Underwood said in advance of the announcement, which comes at Slack’s tech conference in San Francisco on Tuesday.

The Slack administrator at each company controls that shared channel and membership, Slack CEO Stewart Butterfield added.

“KPMG is our auditor and some senior partners would be in shared channels over the years, while some of the accountants would cycle on and off over time,” Butterfield said.

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Like rivals Microsoft (MSFT), Cisco (CSCO), and now Atlassian (TEAM), which just launched its would-be “Slack Killer,” Slack aims to make communications among departments or project teams less reliant on unwieldy e-mail threads.

Slack is also adding support for French, German, and Spanish languages to what had been an English-only product. The additional language capabilities plus support for payment in yen, euros, and British sterling added five months ago should help the product grow internationally although Butterfield noted that 55% of Slack users already come from outside the U.S. Japan is the third-largest market after the U.S. and U.K., he added

Slack now claims 2 million paid users across 50,000 teams, up from 1.5 million paid users in October 2016 and 1.5 million paid seats in January. Overall, Slack says it has more than 9 million weekly active users and 6 million daily active users across paid and unpaid versions.

These figures are important given that more competition from large, well-funded competitors is coming online. For example, Microsoft, which is pushing Microsoft Teams as a Slack Killer, on Monday announced guest access to Teams.

Constellation Research analyst Alan Lepofsky said all the competitors now offer some form of support to people outside a given company. “But what Slack is announcing is the ability for two companies who both use Slack to create a channel that is shared between the two organizations,” he explained via email.

Slack adoption is growing he noted, pointing out what he called the company’s “very impressive” $200 million in annual recurring revenue. However, he added, there is no clear leader workplace messaging.

Microsoft Teams is closely linked with the company’s Office 365 applications, which means it has a potential audience of more than 100 million users. And, Cisco Spark, by virtue of Cisco’s widely adopted Webex web conferencing product, also has a huge potential audience. Other would-be Slack killers include Facebook (FB) Workplace and Google Hangouts Chat.