In AWS Cognito, you can add a user to a group (after first creating a group). A user may belong to one or more groups.
With using the JavaScript SDK (https://github
This API does exist - AdminListGroupsForUser. The reason you're not seeing it is, as the name implies, that the API is currently only available on an admin basis. Cognito doesn't include admin APIs in the mobile SDKs. It would be included in the AWS SDKs/server side SDKs, but it's worth noting that this API does require developer credentials, as do all admin APIs.