If you'd like to stream different music at the same time, you'll need to use separate accounts. You can add up to 5 extra accounts to a family plan, this results in a total of 6 Premium accounts. If you are listening to music with one account, and start music on a different device using the same account, the first device will pause. So it's important to use separate accounts for every user and/or speaker.
Hope this clears things up. Let us know if you have further questions.