Just to add to this since it's high on Google's search.

In the past you could just give exec club the PNR bookings and they would do it.

Now it appears they are a bit more risk adverse/GDPR aware. The other travellers will need to phone CS and add a note to their booking that the GCH is authorised to adjust the booking.

Then you can call GC and ask for a TCP.

Job done. They were also kind enough to allow me to book inbound for my friends although we aren't actually on the same flight. Your millage on that will probably vary (depending on your millage :-))