Words such as anybody and everyone are singular indefinite pronouns. These words can serve as antecedents for other pronouns like him and her — that is, they can be the words to which the simpler pronouns refer. Pronouns that substitute for singular indefinite pronouns should be singular as well.

If the previous sentence sounds artificial to you, look for a natural-sounding, correct alternative. In some cases, you can get your idea across without using pronouns that must agree with the antecedent:

Everyone who wants to sit at the concert should bring a chair.

Or you can revise your sentence to make the antecedent plural:

own chairs.