At the top of the page you will see the option Enable multi select. When this option is selected you will be presented with the screen below.
To assign individual contacts to multiple queues, select the contacts further down on your screen by clicking the plus sign on the top right hand corner of this contact.
This will display the contact you wish to assign in bulk in a highlighted grey shade.
To assign all contacts to multiple queues, tick the Select all contacts option. To assign these contacts to multiple queues, similarly, tick the queues you wish to assign them to or use the Select all queuesoption. Click Assign when you have finished making your selection. You will then see the queues on the right hand side of the page update.
If you select the Enable multi select option but do not wish to use this feature, simply untick this box which will shrink the options displayed above back to the default phonebook screen.
More on the Phone Book