She is only mentioned in Luke, in two verses. This is all the information there is on her.
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Here is Smith’s Bible Dictionary entry for Joanna:
The name of a woman, occurring twice in (Luke 8:3; 24:10) but evidently denoting the same person, (A.D. 28-30.) In the first passage she is expressly stated to have been “wife of Chuza, steward of Herod,” that is, Antipas, tetrarch of Galilee.
The wife of Herod’s steward made her a person to be noticed; I’m sure. Perhaps she was mentioned the way a church today might point out the “wife of the governor’s secretary.”
http://en.wikipedia.org/wiki/Herod_the_Great