I've done emails that were super simple and tied to thanks for whatever advocacy they had done before they officially joined the program. Like a "thanks for doing X. X is just one way you can share your experience and success with Company. I'd like to invite you to be an official member of community/program where you can take part in other opportunities to (your value proposition) and (your reward/recognition pitch). Check out this (link/attachment) and (mechanism for joining)."
Then the brochure or landing page is where you lay out the whole program, detail of your value proposition, rewards/recognition, types of advocacy, etc. I'm building a customer marketing function from scratch in my current role, but here's an example from my previous org: https://www.nice.com/clubs/shine
