DiZ and Riku could have asked her to infiltrate the Organization to capture Roxas.
well, actually, riku was sent by DiZ to capture Roxas, namine was only used to put sora's memories back together.
And there are obviously 14 members, but not 14 tombstones, so whoever the new member may be, her tombstone is not displayed.
well there aren't even 13 tombstones, xemnas didn't have a tombstone, maybe teh girl is in the same ranks as xemnas, like the co-number 1 or something.
It is possible that Namine is the Org member 14, because she could have been part of the Organization but then was demoted or something to just a "witch" and was just used as the one to disassemble Sora's memories. Even though that is highly imporbable, and i myself beleive that it is somehow Aqua's nobody, which could mean that all those keybladers who took part in the BBS, master xehenort, who became teh organizations number 1, ven, who looks like roxas and is probably connected in some way, and that leaves aqua, the DS and terra. i think that Terra maybe just vanished and the DS is probably Xigbar? i don't know what happened to teh DS but the reason that i am saying that Terra just simply vanished is because in the reel shown at TGS, terra says, do not depend on me, just forget me...my freinds. maybe terra disapeared and Aqua lost her heart and turned into a nobody and joined the organization. Aqua may have joined the organization with xemnas, who is xehenort, who is master xehenort, because xemnas does not look like master xehenort at all and xemnas still remembers aqua (because of the scene where he calls her armour "old freind") and then Ven (roxas) joins and aqua and ven talk to each other, aqua leaves roxas wants to know who he really is, he leaves in search of sora, and xemnas is left all alone.