It is never possible to have multiple people on the same bookings who do not have the exact same travel plan (same flights, same dates, same booking class, same bucket, etc). The only thing you can do individually are ancillary services (e.g. pay for a seat reservation or extra luggage for some but not all passengers on the booking).

Anything else, such as people travelling in different classes or with an extra leg, etc will require separate PNRs. If you book everyone under the same and then upgrade some but not all passengers, the PNRs will be split at the time you do that anyway.