Here is how we grow a yoga app: events.
The partner gets an audience. The members get a gift. The app gets new members.
Yoga With Adriene's 30 Days of Yoga is a free daily practice that opens with a welcome video, then one session a day for 30 days, and lets people practise alongside others around the world. A start date gives a person a reason to join today. A finish line gives them a reason to stay to the end.
Picture a small company that sells yoga blocks. It has no audience of its own yet. You tell it that thousands of people who like yoga will hear its name. It gives you a few blocks to give away. The gift costs the company little, and the introduction is worth more than the blocks.
Tell the partner what you will do before you ask for anything. Say how many followers will see it, where, and when.
Write to three related businesses this week. Offer to feature their product for a gift you can give away. Run one challenge. Count how many new members it brought.