Most people that study AI existential risk specifically are studying it due to concerns about AI x-risk. So the list of relevant AI x-risk experts will be subject to massive selection effects.

If instead you want to consider the highest status/most famous people working on AI in general, then the list of signatories here is a pretty good summary. From my flawed perspective as a casual AI enthusiast, Yann LeCun and Jürgen Schmidhuber are the most glaring omissions (and both have publicly stated their lack of concern about AI x-risk).

Of course, the highest status people aren't necessarily the most relevant people. Unfortunately, it's more difficult for me to judge relevance than fame.

Signatory Jan Taallin has founded an x-risk organization focused on AI, biotech, nuclear weapons, and climate change:

https://futureoflife.org/

Who in the world is an expert on existential risk? It's kind of hard to have empirically tested knowledge about that sort of thing.

Pretty sure there are people looking into nuclear deterrence, bioterrorism defense, planetary defense etc. (We didn't have a nuclear war or some bioweapon killing everyone, for example, despite warnings).

There are people studying how previous societies got into existential risk situations, too.

We also have a huge amount of socio-economic modelling going into climate change, for example.

So I'd say there should be quite a few around.