Ignition Interlock is not user specific. If someone else is driving the vehicle, he will be requrie to submit to retests on your behalf. You should train the other drivers in using the ignition interlock and make sure that the driver is completely sober because if someone else drives the vehicle, you will be held responsible for all of his activity.
The Department of Licensing has reassured me that multiple family members may drive a vehicle equipped with interlock ignition. In speaking with local manufacturers and reading the manuals of such devices, I have confirmed that this is achieved because the substitute driver must submit to these tests and becomes your representative. Any violations he makes will be reflected on your record.
On a related note, Ignition Interlock is not necessary on “vehicles owned by a person's employer and driven as a requirement of employment during working hours.” The employer must provide a declaration stating that your work requires you to operate a vehicle owned by the employer during work hours. Here is the link for this application: dol.wa.gov/forms/500025.pdf.