Overlay User Calendars

I'm using my words carefully here because "Team" and "Group" have greater meanings now.

I have a SharePoint calendar currently that has a bunch of events on it.

I want to overlay the personal calendars for everyone in my department (four others).

When I go to Calendar Overlay Settings and create a New Calendar (always feels like I'm doing it wrong), my options are to overlay other SharePoint calendars.

I read elsewhere that Exchange calendars do not work with Calendar Overlay in SharePoint Online.

Is this even possible then?