She's not. Basically, she was just kidnapped and given a new set of clothes, which are just like the ones the Arrancar wear, don't ask why. When Orihime first met Aizen, he placed her under his power. When Ulquiorra confronts here in the soul society he threatens to kill Ichigo, Rukia, etc this Orihime naturally agrees to go and join the Arrancar. This is later explained by Aizen that she never willingly joined them, She was actually under his control the entire time. It is not known if Aizen is aware that she intends to user her powers to make the Hōgyoku as it was before it was created.