Can an iOS app be released as a public beta? I'm wondering if there is an equivalent in the iOS App Store to Google Play's beta testing program Early Access.

https://techcrunch.com/2016/09/01/google-plays-beta-testing-program-early-access-opens-to-more-developers/

https://support.google.com/googleplay/answer/7003180?hl=en

We are currently testing our app with a handful of users and a few small groups via Testflight, but it's unclear to me if we can scale up the testing without just releasing the app to everyone.
To reiterate what I wrote in the comments: Apple does not currently have any "open beta" program like Google Play's Early Access. In terms of TestFlight's scalability, you can invite up to 10,000 testers through the platform (as is listed on Apple's TestFlight web page).