If you have more than one client, you have an option called Tenants when clicking on your login name in upper right corner of the Portal. Clicking on this option will show a list of all clients you have access to.

You can switch between the different tenants by clicking the name of the tenant you want to work in.

If your user has access to one tenant only, he or she will only see the name of this tenant and will not have the option to change tenant.