Legal analysis: the client signs the entrustment contract (agreement). If Zhang San entrusts Li Si to attend the meeting, Zhang San is the principal and Li Si is the principal, the power of attorney should be signed at Zhang San.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 919 An entrustment contract is a contract in which the principal and the agent agree that the agent shall handle the principal's affairs.

Article 922 The agent shall handle the entrusted affairs in accordance with the instructions of the principal. If it is necessary to change the client's instructions, it shall be approved by the client; If it is difficult to get in touch with the client due to an emergency, the agent shall properly handle the entrusted affairs, but shall report the situation to the client in time afterwards.