Each command must start with /viewfinder. Note that you can also just use /view if that seems too verbose to you.

Command Description Required permission
/viewfinder OR /viewfinder help Shows a help page which lists all available commands.
/viewfinder info Shows an info page which displays some generic information about Viewfinder.
/viewfinder start <Player> Puts you into spectate mode and lets you see what the target player sees. If you previously spectated someone, the old spectate job is cancelled and replaced with the new one. viewfinder.command.start
/viewfinder stop Takes you out of spectate mode. You can no longer see what your previous target player sees. viewfinder.command.stop