The wife and I (both BA GGL) had no problems selecting any available seats in the cabin at the time of booking. If not all four of your party are gold, that may be the issue?

However, after linking AA and US PNRs (EWR-PHX on AA F then PHX-SFO on US Y) the AA agent checked us in incorrectly (boarding passes but no e-ticket despite being given PNR) and assigned us seats way down the back - not the exit rows we'd selected.

I went across to US to sort it out and move our seats but the women was adamant that as BA Gold we had to pay for any choice / preferred seats. In her defence, even with Emerald showing on the BPs, when she tried to move the seats, the system asked her for payment.

In the end, she "found" our original e-ticket numbers and seats. However, she again insisted that we must have paid for them. Clearly, US need to better train their agents...