Demi's scenes are expected to be shot mainly with Lea Michele and Naya Rivera although she will also sing with Adam Lambert, who joined 'Glee' earlier this year.
He wrote on Twitter: "@ddlovato I just heard the news! Excited to sing w you girl! (sic)."
Demi will make her 'Glee' debut in the October 3 episode, which will be the second in a two-part Beatles tribute.
The show will then air a memorial episode to 31-year-old Cory Montheith, who died after overdosing on heroin and alcohol on July 13, before it takes a hiatus.
Creator Ryan Murphy said: "We will begin shooting in late August the two shows we had already written, so that people can physically go back to work. We will then do an episode that will deal with the death of Finn's character and follow that with a long hiatus. I don't know exactly when we will come back, and we are trying out best with this attempt at damage control.