Your child's teacher may be able to send the app link to parents in your child's class. Once other parents from your class are on the app, you can start to match up. This is great because these are families you already have a lot in common with, and just need to get connected.
Many schools and school districts are developing resource lists for parents during COVID closures. Mention to your administrators that they should add the app to their resource list. Once other parents in your school are on the app, you can start to match up with those who you are compatible with.
If you have a list of the parents in your child's class, send out the link to those parents and recommend using it. Once other parents are on the app, you can start to match up with those who share even more in common with you than just being in the same class.
Nextdoor is a great platform for sharing ideas with people in your neighborhood. Post a link and a short description of the app on your neighborhood post board. Try posting the link to any other local online post boards that gets the word out to your local community.
Are you a part of multiple parenting facebook groups? Are you interacting with your school via their facebook page? Post a link about the app and drum up interest. Let them know you are a user and why they should join.
Do you have families in your social network on facebook, instagram, etc? Share the app on your pages and encourage them to join. It may be awkward to ask about things like COVID risk level normally, but on the app its transparent, and makes it easy to match up.