Anybody who has ever held the office of President of the United States and attends the first Inauguration that occurs in 2025 in person counts, including whoever is President immediately prior to the Inauguration.

At this moment, there are 6 living people who have held said office: Joe Biden, Donald Trump, Barack Obama, George W. Bush, Bill Clinton, and Jimmy Carter.

Clarification added: they all count no matter why they are there, even if they are there to be sworn in as the next president.