Circling back on this in case anyone else has the same question; we figured it out ourselves.
Adding the following to our .gitignore set us up for working as a team:
shopify.app.toml
extensions/*/dist
The shopify.app.toml file is created automatically upon running npm run dev and is specific to each developer environment. The extensions/*/dist is where the compiled JS files are placed for the extensions. You do need the shopify.extension.toml files in source control but the dist folder is generated on npm run dev and includes a map file which is specific to the local machine.