If you want your current participants to participate more, then research the members who already participate, uncover their unmet needs, and build this into the community.
If you want members who don’t participate to begin participating, survey and interview solely these members. Find out what led them to join the community. Uncover why they don’t participate (do they feel they have nothing to share or get their needs satisfied elsewhere) and adapt accordingly.
If you want more people to join your community (typically customers/audiences you can reach that haven’t joined yet), then survey and interview this group. Find out if they’ve heard about the community, what they’ve heard about it, and why they decided not to join. Is it an awareness issue or a messaging issue? Identify if you need to change the message or do a better job promoting the community.
Each type of research is fine, just be clear about what you’re trying to achieve.