This is what ba.com says:
When you travel as a group [nine or more people on one booking], we’ll allocate your seats free of charge. While we will try to seat your group together, we cannot always guarantee it, particularly if your group travel is split across separate bookings.
To ensure your group is seated together, or if you would like specific seats, you can pay to reserve your seats in advance (for groups booked in the UK and Ireland only).
https://www.britishairways.com/en-gb...ving-your-seat
So paying for an advanced seat allocation may not be an option for a group that is booked in the US.