GarageBand for iOS (iPad): Jam with other GarageBand users:

You can make music with other GarageBand users sharing a Wi-Fi connection. The bandleader creates a jam session, then up to three band members can join the jam session. Playback and recording are synchronized between all devices, so everyone can play and record together as a band. The leader can keep exclusive control of playback and recording, or allow all members to share control.