A third person will always stabilize and grow the relationship between the other two. It’s called a triad, and the more you create, the stronger your network.
Don't care about triads and just want to use S'Up to pair team members? S'Up can be customized to introduce any number of people.
Setup a page for anyone to join your Slack in a couple of clicks. Share the public page that lets users join Slack without an invitation. Optionally, turn on administrative approval with set approval on. Slack admins are DMed join requests that can be approved or ignored.