The plan costs $17.99 for two accounts and three accounts costs $22.99 a month. Unlimited web and mobile plans still cost $9.99 a month (as do additional accounts added to the family plan.
One central account controls the other accounts and assigns logins for family members. You can read all about on the Rdio blog.
You can currently use Rdio on multiple devices but not on multiple devices simultaneously, which makes the family plan a good deal for kids going back to school or couples who want to listen to Rdio on the devices at home or work and would like to share and reduce the cost.