Guest users don't show up in the dropdown component
When you select a people property in a dropdown or multi-select component, we are not able to show your guest users in the list of options. We do receive the admin and members' data. This is because Notion does not provide third-party apps like ours access to the guest data just yet. We hope this changes soon.
From Notion's documentation
However, there is a workaround possible.
Convert your People property to an Email property or Select property on Notion.
Click on Reload Data in NotionApps.
Manually add your desired email addresses as options to the dropdown or multi-select component in NotionApps.