Using Notion in the browser it's possible to get all guests from Settings & members - Members - Guests.
https://api.notion.com/v1/users I can get all members.
But how can I get guests via API?
I have looked for the same thing in the past but it seems there is just no API for getting all the quests as of now.
The only workaround I found is manually exporting CSV of the guests from the SETTINGS & MEMBERESS --> MEMBERS --> GUESTS. But I suppose you already know about this option.
I would recommend submitting it as a feature to the Notion Engineering team. I enough of us do it the priority might get high enough for them to add it.