The service provider typically prepares the Letter of Engagement, be it a law firm, accounting agency, consultancy, or any professional offering services.
For an engagement letter to be binding, there must be agreement to the terms on both sides. The engagement letter is drafted and, in most cases, signed by the firm, therefore there is clear evidence of the agreement of the contents of the letter on the firm's side.
It is in the interests of both the entity and the auditor that the auditor sends an audit engagement letter before the commencement of the audit to help avoid misunderstandings with respect to the audit.
An engagement letter is drafted by the company rendering the service, often with the help of a lawyer. It is than presented to the client, and both parties must sign in order for it to be legally binding.