在场(zài chǎng) - to be present, to be on the scene in Mandarin

在场 (zài chǎng) means to be physically there when something happens. It belongs to the language of accidents, arguments, meetings and police statements, where the point is whether you saw it with your own eyes.

Grammatically it behaves as a verb, so it takes 不 rather than 没 for a general statement: 我不在场. It also works attributively with 的, and 在场的人 is the standard way to say "the people present" or "those there at the time".

The trap for an English speaker is treating 在场 as an all-purpose translation of "present". For being present at a class or a wedding, Chinese prefers 参加 or simply 在. Reach for 在场 when presence is the fact in question, not when attendance is merely being reported.