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.