Williams will put her vocal chops to the test in the live telecast this December.
Harris will not extend in his Tony-winning role.
The annual event will take place on March 31.
The New Group benefit also featured Marin Ireland and Sebastian Stan.
The one-night-only event will also feature Marin Ireland and Sebastian Stan.
The two-time Tony Award winner takes on Kurt Weill, Mayor Bloomberg, Lena Dunham, a coven of witches, and Carnegie Hall.