I'm currently working on https://indiestack.org/ which is a community for independent entrepreneurs. At some point, I want to make it a paid membership. Everyone who signs up before moving to a paid subscriptions gets free membership, as a thank you for the early support.
The problem I'm having is, when to make it paid? A community is interesting in that people won't want to join, let alone pay, without some action inside the community. When do you start charging? 100 users? X amount of posts per day?