You can't stream directly from an iPhone to the Express, but you can use Remote to control an iTunes library on a computer which can them stream the music to multiple Expresses.
The computer is only capable of sending the same stream to them.
Question: Can iTunes send a single AirTunes stream to multiple AirPort Express stations?
Answer: Yes. You can send an AirTunes stream to multiple remote AirPort Express units. The maximum number of remote AirPort Express units is up to six. The actual number that will work in your environment will depend on conditions such as building composition and radio interference, distance to the remote AirPort Express units, and available network capacity relative to your other usage.
AirPort Express frequently asked questions (FAQ)