Season ticket holders get first dibs on their own seats for Football United.
Any extra seats you buy for Football United can't be allocated yet because they might be another season ticket holder's seat.
So if you want to sit with the people you are buying extra seats for you are basically giving up rights to your own seat and have to wait until they do the big seat allocation in a few weeks. That's what I've done (I wanted cheaper seats than my own seat would have been anyway). All we know at the moment is that we will get the "best available" seats in the Bronze Zone.