1 > Get them involved. They're already interested, so this should be easy. Give them a plan to follow.

2 > Have them share their experiences following the plan. What struggles and successes did they have? They can make videos and write posts to share.

3 > Reward them for sharing their experiences. Give them individual feedback.

4 > Give them something to buy. A course, coaching time with you (group/individual), products related to the focal interest.

5 > Give them a reason to share with outsiders. Give them a program, a membership trial, a document, something to entice other people they think would be a good fit to come into the group. Give them reasons to share their own content, the "struggles and successes" video, perhaps, outside the group so others can see it and be filtered in.

6 > Reward them for sharing the content outside the group and bringing new members in.