I'm not sure about the layout of the tables there, but if they have one that specifically seats 6, then booking another reservation for 1 won't do you any good even if you can find that available, unless you're ok with that 1 person maybe setting alone. I'd try calling and/or do the reservation finder as suggested above. Otherwise, the best case scenario is that they seat the 6 of you at a table that supports 7 or 8 and then you can easily add the extra person, but that's a crapshoot.