“You can use the free version of Slack for as long as you like andthere’s no limit to the number of members who can be invited.”
That’s what Slack’s pricing page reads once you get started. In all their PRs and advertisements', Slack has time and again claimed that their free tier offers an unlimited number of users.
In short, Slack unapologetically mentions that there is no limit. You can add ‘as many people’ as you want to.