We use the reciprocal pronouns each other and one another when two or more people do the same thing. Peter and Mary helped each other. Peter and Mary helped each other. = Peter helped Mary and Mary helped Peter. We sent one another Christmas cards. = We sent them a Christmas card and they sent us a Christmas card. They didn't look at each other. = X didn't look at Y and Y didn't look at X. Themselves is a plural pronoun. Since it is used reflexively, it is used in contexts where the subject of a sentence is also its object. And since it is plural, it refers to more than one person. Themselves is a reflexive pronoun that refers to a group of people.