Margaret was a companion of the Doctor.
She was captured by the Nine and placed in a prison along with various other companions. She could be heard bemoaning the fact that she was not young any more, and begged to be let out. (AUDIO: Companion Piece)
Behind the scenes
Margaret has not appeared in any other stories and Companion Piece never states which Doctor she travelled with. The corridor she was kept in implied she may have been a companion to the Fourth Doctor.