Here are some ideas.
What Does It REALLY Take To Start and Launch a Membership”
Share the idea that you are thinking about doing this (and let people know where the idea for your membership came from. It would be great if you have documented this part).
Cast a vision and do a “Founding Member Launch”. It is where you invite subscribers/followers into your membership before you feel ready. Letting them know that it’ll be a work in progress. But that you all will be building it together. The call to action would be “Hit reply and let me know that you would like to be a founding member.”
This could even be done even with a Facebook post (Stu did that for one of this launches). And, in that case, invite people to send you a DM.
Follow-up when people respond - “It’s going to be launched at this time. If you’re interested here is where you can go.”
Use this launch to create momentum and collect success stories.
Building a community organically: https://blog.tribe.so/build-community-organically-from-scratch/32
Think up a community persona (as if the community were a person)
Find members for whom that resonates (and build trust with those people, so that they feel confident that joining will add value for them)
Repeat (to scale)
Create exclusivity within the group when you begin scaling
Also, in case you are also curious about helping to build an audience to start with, Stu tells how to build an email list from scratch too.
How do I grow my email list?: http://www.youtube.com/watch?v=5gECXJqelB012
Isolate a market problem
Create list-building events (like webinars, or a white paper, or an epic blog post with a compelling lead magnet, Facebook live sessions, whatever)
Repeat (do it on a regular basis)