I created a new group through Admin control panel and use Directory API to add new members in bulk, using batch-update, to the tune of 1000 users.
The documentation states that it should be valid.
You're limited to 1000 calls in a single batch request. If you need to make more calls than that, use multiple batch requests.
The script ends without error, however not all users are showing. Sometimes 300 get added on each execution, sometimes 150. I have to refresh it again and then more members are being added in chunks.
I wanted to clarify from experts if there are any restrictions to this approach using API? For example, may be, you can't add more than X number of users to a newly created group and perhaps you have to wait 24 hours.
as the documentation states a batch can contain a max of 1000 request however I recommend going a bit lower say 900
You're limited to 1000 calls in a single batch request. If you need to make more calls than that, use multiple batch requests.