Coming to Slack, the communication service for teams and businesses: The ability to share channels with other companies, a new beta feature called "shared channels."

The big idea: Teams of vendors, clients, partners, etc., will be able to communicate and collaborate more normally within Slack, instead of inviting specific individuals to be guest accounts within one company's Slack or using other tools like email.

The new feature will almost certainly cut down on email between companies that share channels, just as Slack has famously cut down on internal email for any company that uses it even remotely the right way.

More broadly, it allows Slack to command more control over the way people communicate and get work done.

And in that context, it's an obvious feature to add. So why did it take so long?

Slack now has enough traction -- six million daily active users and two million paid users across 50,000 paid teams, including 43 percent of the Fortune 100 --to make cross-company sharing practical for many -- if not most -- of its users.

Slack says about two-thirds of customers already have guest accounts on their teams; it's been testing the new shared channels feature with a handful of companies this summer.