No. WWOOFers are responsible for arranging their own travel requirements. If traveling from another country, this means securing their own visas and travel documents. Hosts are not required to assist a WWOOFer with any documentation for their visa applications, and WWOOFers are encouraged to secure their visas before joining as a member.
If you are asked about providing a letter or other documentation, please direct WWOOFers to the visa information.