Slack does this as well. It's a genius idea from a business perspective. Normally IT admins have to go around asking users if they need the service (or more likely you have to request a license for yourself), regularly monitor usage, deactivate stale users etc., all to make sure the company isn't wasting money. Slack comes along and says - don't worry, just onboard every user at the company. If they don't log in and send at least N messages we won't bill them for that month.