Why not ask outsiders to give their expertise? It’s an effective way to grow your online community.
You can approach people on relevant issues to your community and ask them to share their outsider’s opinion. Be specific about why you need them, what skills they have that you need and the problem you need their help with. Maybe they also know some friends who can help?
When they join, introduce them to the group. Shine a spotlight on them with an interview and encourage members to treat them warmly. Celebrate their successes and input. People love to feel valued.
For bonus points, coach your members to reach out to outsiders on their own initiative.