When using Forms connected to SharePoint, there are different workarounds to Filter or limit the selectable Choices on a People Picker control. One of the workarounds involves building a custom People Picker with a Combo Box control and setting the Items property with the Office365 user connector. Another workaround involves limiting the selectable people in the Combo Box control using a SharePoint security group.

Another workaround, which I have not seen documented online, has been detailed below for reference.