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Rdio doesn't have that many details about the plan yet, but will be revealing more about it during the next few weeks.
Although it is wholly possible to share a streaming account -- just give your password to another person -- you can't currently play music from two locations at the same time using Rdio. (It's also illegal to share passwords for streaming services -- if you live in Tennessee.)
As far as we know, Rdio is the only on-demand service to offer a family plan. Still, it's possible to listen to Spotify -- for one -- from multiple devices at once (provided you have premium and use offline syncing), so we're curious to see what other trappings Rdio offers users.