Create a TestFlight beta group, invite testers, and assign builds to the group via the App Store Connect API

Documented steps

1. Create the group: POST https://api.appstoreconnect.apple.com/v1/betaGroups with {"data":{"type":"betaGroups","attributes":{"name":"QA Team","isInternalGroup":false,"publicLinkEnabled":true,"publicLinkLimitEnabled":true,"publicLinkLimit":500},"relationships":{"app":{"data":{"type":"apps","id":"<APP_ID>"}}}}}. The app relationship is REQUIRED; the attributes are optional.
2. curl: curl -X POST https://api.appstoreconnect.apple.com/v1/betaGroups -H "Authorization: Bearer $JWT" -H "Content-Type: application/json" -d '{"data":{"type":"betaGroups","attributes":{"name":"QA Team","isInternalGroup":false},"relationships":{"app":{"data":{"type":"apps","id":"<APP_ID>"}}}}}'
3. If publicLinkLimitEnabled is true, publicLinkLimit must be an integer between 1 and 10,000.
4. Invite a brand-new tester and join them to the group in one call: POST /v1/betaTesters with {"data":{"type":"betaTesters","attributes":{"email":"tester@example.com","firstName":"Ada","lastName":"Lovelace"},"relationships":{"betaGroups":{"data":[{"type":"betaGroups","id":"<GROUP_ID>"}]}}}}. Only email is required.
5. Bulk-add EXISTING testers to another group via the linkage endpoint: POST /v1/betaGroups/{id}/relationships/betaTesters with {"data":[{"type":"betaTesters","id":"<TESTER_ID_1>"},{"type":"betaTesters","id":"<TESTER_ID_2>"}]}. Returns 204 No Content.
6. Assign builds so the group can install them: POST /v1/betaGroups/{id}/relationships/builds with {"data":[{"type":"builds","id":"<BUILD_ID>"}]}. Returns 204. Confirm the build's processingState is VALID first.
7. Remove testers from a group (revoking that group's build access without deleting the tester): DELETE /v1/betaGroups/{id}/relationships/betaTesters with the same linkage body shape. Returns 204.
8. Remove builds from a group: DELETE /v1/betaGroups/{id}/relationships/builds with {"data":[{"type":"builds","id":"<BUILD_ID>"}]}
9. Internal groups (isInternalGroup true) may only contain existing App Store Connect users on the team holding an eligible role (Account Holder, Admin, App Manager, Developer, Marketing). External groups accept arbitrary invited emails but require the assigned build to pass Beta App Review first.
10. Per Apple's TestFlight documentation, up to 100 App Store Connect users can be internal testers and up to 10,000 people can be external testers per app.
11. To fully delete a tester rather than just unlink them, use DELETE /v1/betaTesters/{id}.
12. Official docs: https://developer.apple.com/documentation/appstoreconnectapi/betagroup | https://developer.apple.com/documentation/appstoreconnectapi/post-v1-betagroups | https://developer.apple.com/documentation/appstoreconnectapi/betatester | https://developer.apple.com/help/app-store-connect/test-a-beta-version/testflight-overview