And just at that very moment,
@JuhaL happened to start Zwifting, so I was able to race upstairs to my bike (I'm on my lunch break from WFH, so it's allowed) and get you some screenshots! Sorry to Juha for using you as an example - I'm not stalking you, honest!
We'll start with the usual opening screen to Companion, we can see Juha is Zwifting and it would be nice and easy if we could watch his ride from here, but unfortunately it's not as simple as that...
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As soon as your version of Zwift starts, Companion will then automatically take you to the map screen. You have to make sure that you choose the same World map as the person who you want to view, otherwise this won't work. Now tap the More button at bottom right.
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From here, tap on Activities
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Now annoyingly, this screen only shows completed activities, not ones "In Progress", so you will have to scroll down the list until you find the previous activity that this person did.
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Once you find one, go into it, then you can tap on the photo/logo of that person's name at top left as highlighted here:
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Finally you've now got to their profile (the long way round) and you can see the Fan View button (sometimes you might need to scroll down a bit).
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Give that a tap and all of a sudden, you'll be switched to viewing their bike on your PC/iPad/AppleTV (assuming you're on the same map, otherwise the button does nothing). This works even if they are currently in a race or group ride that you hadn't joined.
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Hope this helps. I'm sure there's probably a quicker way to do this, but this is the way I've managed to get it to work, so it's what I'll use until someone shows me something better!