In truth, Orihime was just a pawn in Aizen's plan to lute the captains of Gotei 13 as well as Ichigo's group into Hueco Mundo so he could seal off all ways out and trap them there. At this point, the ones trapped are Ichigo, Sado, Rukia, Renji, Uryu, Kenpachi, Yachiru, Mayuri, Nemu, Unohana, Isane, Byakuya, Neliel, Dondochakka, Pesche, Grimmjow (who is still seriouly injured), Ulquiorra (who's still trapped in a pocket dimension by Grimmjow's Caja Negación), Gantenbainne (who was mysteriously healed by Unohana), and, who's been sealed in the fifth tower of Las Noches, Orihime. As Aizen has no further use of Orihime, he left her in Las Noches while he went to Karakura Town to create the King's Key with the Hougyoku. Urahara had planned this all along, however, and now the remaining forces of the Gotei 13 have arrived in Karakura Town to fight the final battle against Aizen!
Current known roster:
Commander General Yamamoto
First Division Vice-Captain Chōjirō Sasakibe
Second Division Captain Soi Fon
Second Division Vice-Captain Marechiyo Ōmaeda
Seventh Division Captain Sajin Komamura
Seventh Division Vice-Captain Tetsuzaemon Iba
Eighth Division Captain Shunsui Kyouraku (YAY!)
Tenth Division Captain Toshirou Hitsugaya
Tenth Division Vice-Captain Rangiku Matsumoto
Thirteenth Division Captain Jūshirō Ukitake
Third Division Vice-Captain Kira Izuru - He must want to help bring Ichimaru back to his senses.
Fifth Division Vice-Captain Momo Hinamori - She wants to bring Aizen back to the way he was before.
Eighth Division Vice-Captain Nanao Ise (Please let Nanao-chwaan be in this fight too~!) - You can't have a Kyoraku without a Nanao!
Ninth Division Vice-Captain Shuhei Hisagi - After all this time of contemplating, would he refuse a chance to bring Tousen back to his senses?
Eleventh Division Third Seat Ikkaku Madarame - His captain and vice-captain are trapped in Hueco Mundo. He's the most powerful fighter in Eleventh Division available right now!
Eleventh Division Fifth Seat Yumichika Ayasegawa - He always follows Ikkaku, why not now?
Former Twelfth Division Captain Kisuke Urahara - He set up this master plan, he should be the one who participates as well, yeah?
Thirteenth Division Third Seats Kiyone Kotetsu and Sentaro Kotsubaki - Wherever Ukitake goes, they go.
Former Sealth Force General Yoruichi Shihouin - Yoruichi is always by Urahara's side anyway, but she'll probably assist Soi Fon.
Former ??? Division Captain Isshin Kurosaki - Would he dare let the Espada take away his home while his son is away!? In the name of his deceased wife, he won't let it happen, even if it means everyone else discovering the truth!
"The Last Quincy" Ryuuken Ishida - Well, seeing as Uryu took Ryuuken's Quincy Cross, I doubt he'll be as powerful as he was (unless he had a spare), but if he can fight to protect Karakura and show up Isshin, why not take the chance? (I can see a whole "kill count" contest going on between Isshin and Ryuuken a la Lord of the Rings)
Famous Spiritual Medium Don Kan'onji - Incredibly weak, but doesn't actually realize it yet.
Karakura Red Karin Kurosaki - Always helping out Kan'onji so he doesn't get himself killed.
"Real" Karakura Red Jinta Hanakiri - His power is referred by Urahara as that of a vice-captain, but you never know.
Karakura Pink Ururu Tsumugiya - Her power is much greater than Jinta, but she's also unstable around powerful hollows and arrancar.
Urahara Shop Assistant Tessai Tsukabishi - His knowledge of healing Kidou could greatly assist the battle.
Last Bount, Koga Go - He's anime only, which is why it's unlikely. However, he would be a huge asset to the battle.
High School Students Keigo Asano, Mizuiro Kojima, and Tatsuki Arisawa - Urahara could have trained them to focus their energies (well, at least Keigo and Tatsuki) to help in the fight, even a little.