Multi-tenant support allows certain users to belong to a subset of tenants, for organizational purposes. This feature allows users to view and manage the tenants they need to access. For example, if two of your teams have a tenant and you want to grant a manager access to both.
In order to configure multiple tenants, you must belong to multiple tenants or be a tenant admin. You'll only see the tenants you belong to and if you're without multiple tenants, then the tenant options will not be clickable. You will be able to view all the users of all the tenants you belong to and will not run into duplicates as users are listed only once.