Here are some steps you can take to expand your network.
The ultimate networking aid
There are plenty of websites designed to connect podcasters like you with people who have something to share. Podchaser Connect and Podcast Guests are excellent places to start.
Six degrees of separation
Ask your guest which person in their network you should interview next. When they name someone, ask for a connection. This can be done after the show or, even better, as part of your interview.
Let people come to you
Create a guest request page on your website where listeners can apply to be on your show. You don't even need a website - a Google form and a simple URL redirect works just as well.
Don't ask, don't get
Take things back to basics and put a short post on social media asking for guest connections. It's easy to overcomplicate networking strategies, so don't lose sight of the basics.
A group effort
There are a multitude of online communities dedicated to all things podcasting. Join some Facebook groups and you could not only get some valuable advice, but you're likely to pick up some guest opportunities as well.
Reach out to other podcasters in your industry and ask to appear on each other's shows. If you're looking for guests, chances are other people are as well. So this is an excellent networking tactic that also does wonders for audience growth.
A novel idea
When authors launch a new book their publisher will encourage them to get on as many interviews as possible to promote their book. Look for upcoming book releases and contact the author to get ahead of the curve.