A joint contest is when a guest offers something up to your audience, usually at no charge to you. They will promote the offer to their audience because their audience may end up winning this product or service, but when the subscriber enters their email to enter the contest, you are growing your email list. You would also alert your email list of the contest, which puts the guest’s product or service in front of your audience.
I did this with Travis Brown and his PodDecks. He was a guest on my show, and he sent me two free stacks of his complete PodDecks: one to keep for myself and the other to offer for free as a contest. So when he was a guest on the show, we mentioned the contest, and we also both emailed our respective email lists about the contest. I grew my email list, and he got publicity and some new buyers as well.
Also, a win-win.