The Table Occurrence for People is listed in Portal setup, but it's greyed out and not available because it is not related to Customers, only to Customers 2 and your current layout refers to Customers, not Customers 2.
Every layout specifies a specific table occurrence in Layout setup | Show Records From.
You'll be able to create a portal to People if you either link it to Customers (easiest to do) or redirect your layout to Customers 2 by selecting Customers 2 in Show Records From for this layout. If you take the second option, every field on the layout has to be respecified to come from Customers 2 and any other references to related tablers, such as another portal, will no longer work unless you create additional table occurrences for them, link them to customers 2, then update the references...
It worked perfectly, thank you for the clear response and for the speed of the response.