The U.S. Trustee Program website addresses this....
The Judge determines whether a U.S.Trustee is needed in Chapter 11 cases. (A U.S.Trustee is always appointed in a Chapter 7 case). Once appointed, the U.S.Trustee may appoint equity committees. See below....
Section 1102 of the Bankruptcy Code directs and authorizes the United States Trustee to appoint an official unsecured creditors' committee and provides the United States Trustee with the discretion to appoint additional committees, including equity security holders' committees. The United States Trustee must endeavor to appoint a committee of creditors holding unsecured claims "as soon as practicable after the order for relief. . . and may appoint additional committees of creditors or of equity security holders as the United States trustee deems appropriate." 11 U.S.C. § 1102(a)(1). "On request of a party in interest, the court may order the appointment of additional committees . . . if necessary to assure adequate representation of creditors or of equity security holders." 11 U.S.C. § 1102(a)(2). "On request of a party in interest in a case in which the debtor is a small business [as defined in 11 U.S.C. § 101(51C)] and for cause, the court may order that a committee of creditors not be appointed." 11 U.S.C. § 1102(a)(3).