Most podcasts like you're describing start by using the audiences of their guests. This makes it tricky, because you're effectively asking someone to come on your show to leverage their audience for your growth.
That's still a value proposition for the guest, however, because it's back links, external content, and more credibility to their own audience. (Assuming they don't already have a podcast.)
Aside from leveraging your guests' platforms, you can also use standard marketing tactics to create your own network: guest blog, get active in the community, and do whatever else you can to generate backlinks and buzz.
I'd be happy to go over strategies that have worked for me and my clients if you need a starting point. Just let me know.