Yamaguchi, who left the Seminoles with one year of eligibility left to join Swedish powerhouse Umea, was selected third by the Beat in the 2009 WPS International Expansion Draft. Bachmann also played for Umea.
"Mami is a very technical and versatile player," said Atlanta coach Gareth O'Sullivan. "She is extremely clever in possession and will bring great experience to the team. We expect her to have an immediate impact in the WPS."
As for the 18-year-old Bachmann, O'Sullivan said, "Ramona is one of the most exciting young players in the world. She is confident on the ball, has good pace, and is very direct in the way turns to take players on."
Brazilian Marta also starred at Umea before joining WPS's Los Angeles Sol.
"In seeing the number of players who are now leaving Sweden for the American league, I think right now the best players are in WPS," said Bachmann, "and I want to play both with and against the best players in the world."