Noble Team, the squad you'll be joining in Halo: Reach, only has six members. For the upcoming San Diego Comic Con, however, Bungie has drawn up a seventh member, available only in action figure form.
Yes, designed by Bungie's artists solely for the purposes of selling a toy at a comic convention, Noble Seven has an assault rifle, frag grenades and a "custom armor loadout". Happily he does not have a skull painted on his visor, but sadly he does not have a gimpy little robot arm, either.
In theory he'll be available only at Comic Con, but in practice you should be able to find him fairly easily online, albeit at a premium price.