The normal rule is that parties who enter into a contract to arbitrate are bound by that agreement and must arbitrate the claims arising from the contract as set forth in the contract. However, in California there are other ways a nonparty to the contract can be compelled to arbitrate. They are: (1) incorporation by reference; (2) assumption; (3) agency; (4) alter ego or piercing the corporate veil; (5) estoppel; and (6) third-party beneficiary. Thus, just because you did not sign the contract does not mean you are exempt from arbitration.
Always consult an attorney to determine what your rights and duties are and whether you can be compelled to arbitrate a claim.