Yes, anyone can come with you, but they also need to provide valid ID. You really don't need anything else other than your passport (or other form of ID) and your appointment letter.
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| Applied with: Lawyer in CA | EWI | Packet sent: 10/11/2012 to Phoenix, AZ Lockbox | Delivered: 10/12/2012 | Signed off by: E. Arndt | G1145 Notification: 10/17/2012 routed to California| I-797C: 10/20/2012 | Biometrics: R'cvd 10/19/2012 for 11/05/2012 | Walk-In: 10/22/2012 | Approved: 5/3/2013 | EAD Sent: 5/7/2013 | EAD Rcv'd: 5/10/13 | SSN card: Issued 1990 |CA DL: Issued 2005