By default, Users belong to the instance they were created in. However, they can be assigned to multiple instances, if required. Ideally, the global administrator or his equivalent should be given such a privileged access to multiple instances.
- Go to Manage→Users.
- Click on the action button of the User you want to assign instances to.
- Select Instance Assignment from the drop-down menu.
- Select the instances you want to assign.
- Click on Save.