Does the response that comes back from the API call account for multiple users added at once? In the documentation it appears that only one object with two properties comes back. Are the users added comma separated in the object that comes back?
Which App Type (OAuth / Chatbot / JWT / Webhook)?
Knowing the API endpoint/s can help us to identify your issue faster. Please link the ones you need help/have a question with.
The Request object accepts an array of objects representing each member to be added to a group which is different from the Response in that it is only one object that is returned.