How do you build your community before launch if there's no marketing allowed in most groups?

To help grow our product community, we look for questions people have regarding the problem our tool is solving. That way, we can recommend our tool in a trusting, reliable way as it is helping them solve a problem they have/ answer their question.

Definitely embed yourself in the community by engaging normally. People don't want those who just drop by and spam their community with stuff they didn't ask for. By being with the people you are trying to serve, you can indirectly learn about them and their needs. And when the opportunity comes up, you can talk about what you're building.