If an order has a customer (it might not), then the customer is already invited. It makes no sense to re-invite an already active customer.
If the customer is part of the order, but is not enabled, and not invited, then it seems to make sense that you could programatically send them their invite by enabling their account with the notification set to true.
I am curious, it has been awhile since I played with customer records, but why are you getting customer records with orders that are not customers that can log in and make future purchases on their account? Having customers that can purchase things and then never log in again, makes no sense and sounds like a zinger of a bug.