Are there any other known members of Anbu: Root, apart from Sai and Danzou? In Naruto, apart from Sai and Danzou, which other characters were members of the secret subdivision of Anbu, Root?
At one point, Yamato was a member of Root. This is true in both anime and manga. Kakashi was also a member of Root, but this is only true in the anime. Yamanaka Fuu and Aburame Tatsuma, the two ninja Danzo brought to the Five Kage summit was also member of Root. For a full list of Root members, see the wikia page.

Credits to Ero Sennin for pointing me the Root wikia page.