If the client has offices in multiple locations, you can add them using the Locations tab within the main list of Clients. Users at a client can then be associated with a specific location.
Simply use the Add Location function, adding a new name for the new location along with other address and contact information. A Client may have more than one location, you can add as many as you need.
The location details will be displayed in the sidebar when viewing a ticket from a User associated with this location.