For starters, have your trigger action be When an Item is Modified.

Then, do the action Get Changes for an Item. It doesn't tell you what the change was, just a true/false if that field was modified. So you can halt early if it wasn't changed.

If you want this to be easy, just email everyone in the modified field.

But, if you really want to email just the newly added person, it's going to get hard. There is no Sharepoint connector that can get the previous version of an item. Instead you'll need to use the Send HTTP Request to SharePoint action.

Here is an article on how to build that. https://community.powerplatform.com/forums/thread/details/?threadid=f108658f-2f77-49f8-811e-d24043c11cb3

Then it should be fairly easy to filter array to in list now, but wasn't in list before.