How to add User into the Group and remove User from the Group using Calrizen REST API?

Create a new object of type GroupMemberLink and set the "container" field to the ID of the group and the "member" field to the user id.

something like PUT /services/data/GroupMemberLink 

{

  "container": "/group/xyz12345",

 "user": /user/someUserId123"

}