Who is the Maenad chorus in the play Antigone?

The simple answer here is that the chorus in Antigone is not composed of Maenads. To help you as you study Greek tragedy, this answer will unpack your question and address the role of the chorus, the Maenads and where Maenads appear in Greek tragedy, and the chorus in Antigone as separate issues.

Chorus: One element of both classic Greek tragedy and Old Comedy (but not the New Comedy of Menander) is the presence of a chorus, a group of 12 or 50 male actors (sometimes dressed as women) who sing and dance as a group, and are normally led by a choragos, or chorus leader. 

Maenads: The Maenads (also known as the Bacchae) are female worshipers of the god Dionysus (or Bacchus). They form the chorus of Euripides' play The Bacchae, but do not appear in Antigone. 

Chorus of Antigone: The chorus in Sophocles' Antigone is comprised of the elders of the city of Thebes. They are not Maenads.