How to sit together on train from paddington to Oxford

It may be that they have sold all the reserved seats - not all seats on the train are reserved.
This may help:
https://www.gwr.com/your-tickets/seat-reservations

Someone may correct me on this, but I don't think you can select individual seats, rather you choose options, like 'face forward' and 'table seat'. Having traveled many times as a smaller group (4 people) we've always been seated together. If for some reason you're not, you can always move around on the day to any unreserved seat, or for that matter a seat that is reserved but unoccupied because the passenger has failed to board.

Check out this site:>https://www.gwr.com/your-tickets/ways-to-save/cheap-tickets/groupsave

I don't think that the trains which terminate at Oxford have assigned seating. I think that is only on the IET trains which continue to Worcester or Hereford hourly via Moreton in Marsh.
The others are really just commuter trains.
The man in seat 61 has seat diagrams for the IET sets but then the next question is whether it is a 5, 9 or 10 car set.
The IET's have very few tables, mainly airline style seating. But you can't pick your seats off a diagram- merely see where your seats are.
Booking seats after ticket purchase you can only do 4 at a time - just done a sample free reservation booking ( on the 0750 from Paddington next Tuesday- I got A27, 28, 31 and 32). I specified round a table and I got exactly that.

Shame there isn't one more of you as with 10 you could have booked through GWR Group Travel who would have ensured you could have sat together.

The least busy in general will be Oxford terminators with 9 carriages but there aren't many of those. The 10:22 on a Saturday is usually booked to be one though.

All the trains to Oxford from Paddington are reservable but there is an unreserved carriage or part of one (G on 9 carriages, the standard part of D on 5 carriages).