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: Article 925 of the Civil Law of People's Republic of China (PRC), the contract concluded by the trustee with a third party in his own name within the scope authorized by the principal, and if the third party knew the agency relationship between the trustee and the principal when concluding the contract, the contract directly binds the principal and the third party; However, unless there is definite evidence to prove that the contract is only binding on the trustee and the third party.