I think there's also a very important step which is that often these successful communities have value even if others don't join -- what I've heard previously described as "Single Player Mode".

Instagram had filters and photo editing -- and could then also post to Facebook. Even if you knew no one else on Instagram, there was a reason to use it.

Github is still a great UX for Git. You can host your own repos and work with your own team, even if you never look at others.

Why do people want to join first?