The current The Confluence Cloud REST API endpoint (POST /wiki/rest/api/group/userByGroupId and /wiki/rest/api/group/userByGroupId ) only supports a 1:1 relationship — one user per request.
There is no equivalent bulk operation for adding (or removing) multiple users to/from a group in a single call.

With Confluence Cloud now supporting up to 250,000 users, administrators and apps frequently need to manage group membership at scale. 

Extend the add/remove group membership endpoints to accept an array of user account IDs in a single request body.