Betazed is a Federation member world, and home to the Betazoids.
In 2374, the Dominion invaded Betazed while the Tenth fleet was on a training run. (DS9 episode: "In The Pale Moonlight")
Captain David Golds daughter was visiting Ambassador Lwaxana Troi just as the Jem'Hadar attacked. Her assistant, Homn, was killed when the Dominion fired on the surface. (novel: Tales of the Dominion War)
The inhabitants however fought back, and eventually ridded the world of Dominion forces, but not until after suffering heavy casualties themselves. (TNG novel: The Battle of Betazed)