Whomever and whoever are pronouns that mean the same thing: Whatever person or no matter who. In formal speech or writing, whomever is the form of whoever used when it is the object of a verb or preposition.

They equals whoever, them equals whomever. Like the objective pronoun whom, whomever works only as an object of a verb or preposition. Everywhere else, the correct pronoun is whoever (which, like who, is a nominative pronoun).