26 C.F.R. § 31.3121(o)-1
The term "crew leader" means an individual who furnishes individuals to perform agricultural labor for another person, if such individual pays (either on his own behalf or on behalf of such person) the individuals so furnished by him for the agricultural labor performed by them and if such individual has not entered into a written agreement with such person whereby such individual has been designated as an employee of such person. For purposes of this chapter a crew leader is deemed to be the employer of the individuals furnished by him to perform agricultural labor, after 1956, for another person, and the crew leader is deemed not to be an employee of such other person with respect to the performance of services by him after 1956 in furnishing such individuals or as a member of the crew. An individual is not a crew leader within the meaning of section 3121(o) and of this section if he does not pay the agricultural workers furnished by him to perform agricultural labor for another person, or if there is an agreement between such individual and the person for whom the agricultural labor is performed whereby such individual is designated as an employee of such person. Whether or not such individual is an employee will be determined under the usual common-law rules (see paragraph (c) of § 31.3121(d)-1 ).
26 C.F.R. §31.3121(o)-1