The Concorde group tour had to be finalized several days ago and they increased the maximum on 1 tour to 16 (from 14) just for us. There are only 16 seats available for any given Concord tour. The museum will allow me to join the group based on my offering to stand next to or behind tour guide.

However, there is no group rate for Concord tours, everybody pays $11. You can try to buy a ticket for a (different) Concorde tour online or at the time of our visit, I don’t know if and when Concorde tours sell out, they last half an hour and are scheduled every half hour.