If you use "everybody", so the posesive is his or her because it is singular. "All the guest" is plural, so you can use "their"
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lagatawWhich one is standard?Purists insist on "his": Everybody took his seat. Ordinary people use "their", with either the singular or the plural. I would use the plural. Everybody took their seats.
"Everybody took their seat."
"Everybody took their seats."
"Everybody took his seat."
CalifJim"All the guests took their seat."They're both the same. Maybe you meant to make one plural. I would use the plural, as above. All the guests took their seats.