User access to instances is determined by the administrator on the Edit User screen under Instance Access.
Once a user is added at a parent instance and given access to child instances at the parent level, this does not grant the user the same access in the child instances that he/she has at the parent level. An Administrative user with access to manage users and roles in the child instance must assign the user a role and level access in the child instance.
If you create a new user in a child instance, you will not have access to create a user in a parent or sibling instance. If you create a user with a login that already exists in a parent instance (e-mail address), the user accounts will become linked.
You cannot remove a user from their current instance. Attempting to delete a user who has access to more than one instance generates a warning message.
A non-administrative user can choose the preferred default instance on the profile page. This is the instance that is selected when the user logs in. This option appears only for users who have access to more than one instance.
User and Instance Name in Header
The instance you are viewing is displayed next to the navicon. For users who have access to multiple instances, the instance name acts as a drop-down menu. You can use the menu to select which instance you want to view.