The Weakest Becomes The Strongest
Weak. Useless. Dead weight.
That's what she was in all of Konoha 12's eyes. Well , minus Hinata , Yamato and Sai. But yeah , other than that no one cared and that made her an empty shell. That empty shell was a perfect weapon for Danzo and he offered her to join ROOT ANBU. Having no other job than being hospital head , she quit her hospital job and joined the ANBU on the catch that she would be partners with Sai. Agreeing , Danzo let her be partners with Sai. This whole event happened when all of Konoha 12 , minus Sai and Sakura , went to retrieve Sasuke. Why they left Sai and Sakura ? Because sakura was a burden and Sai wanted to be in the company of the girl that first excepted him as a friend. Hinata tried to stay but all of the sensei and the genin forced her to go. Everything went downhill for Konoha 12 when they returned to see Sakura gone.
Also she doesn't only become ANBU , lotsa stuff happens.