Most frequently there are questions on how to add/delete groups/users from Ambari. This document describes how to add/delete ambari's local users and groups (Delete applies to LDAP users/groups as well).
This document should give a consolidated list of rest calls to manage users/groups.
Following are the steps for end-to-end creation/updating users/groups using ambari REST API.
12) Removing a user from the users list in a group :
To remove a user from a group, request a PUT with request body having a list of users that should be included in the group (Exclude ones that need to be deleted)