This document discusses pronoun agreement and reference. It provides rules for pronoun usage:
1) A pronoun must agree in number with the word it replaces - singular pronouns refer to singular nouns and plural pronouns refer to plural nouns.
2) A pronoun must clearly refer to the word it replaces without ambiguity.
3) Indefinite pronouns like "everyone" and "-body" words are always singular. Collective nouns can be either singular or plural depending on whether the group is functioning as a unit or individually.