tHE SAME KEY-SETUP YOU USE TO SWITCH FROM PERSON TO PERSON WHILE IN SPECTATOR MODE...oops, sorry about that.
Some people change it, but I believe its your default primary-fire key... just keep hitting it until you get to the specific player you want. Hit your alt-fire to go directly back to yourself.
This is my using it to view from one of my bots:
You can shift your perspective so as to see what the bot specifically is looking at from its perspective, and then out again; that's F9 and F10 for me currently. It's especially fun to do this with monsters, pop into their viewpoint to see what they're looking at.
I know there's a way to do it with the NUM-LOCK keys too, done it accidentally and freaked out when my viewpoint switched to a bot.