(field workers were employed by farm labor contractors to perform work for defendant farm; under facts and circumstances, court held that contractors were independent contractors of farm, so plaintiffs not employees of farms through employment with contractors; under joint employment analysis, however, employees of contractor were jointly employed by defendant farm under Migrant Seasonal Agricultural Workers Protection Act because the worker is so economically dependent upon the agricultural employer as to be considered an employee; court also held defendant farm negotiated workers' wage rate and workers paid directly from funds received by defendant farm, so plaintiffs employed under state law).