Firebase app distribution: How to add testers to tester group automatically with command?

I am using the below Firebase command to invite testers to my Firebase app as per the documentation here: https://firebase.google.com/docs/app-distribution/ios/distribute-cli

firebase appdistribution:testers:add anothertester@email.com moretesters@email

We maintain two tester groups for these testers for our app. The above command only adds testers to our app's portal but not to the tester groups.

Is there a command/way to add the testers to a particular tester group automatically?

Things I have tried so far:

1. Tried using --group and --groups flag which are used in firebase appdistribution:distribute command but got the error: unknown option '--groups'
2. Also tried using the Firebase App distribution API POST call and getting the same result:

POST /v1/{project=projects/*}/testers:batchAdd