Due to the high volume and complexity of its work, Congress divides its tasks among approximately 250 committees and subcommittees. Most subcommittees are created to hold hearings, mark up legislation, and report measures to their full committees for further action. Subcommittees work within guidelines established by their parent committee, so the number and autonomy of subcommittees varies.
You can find subcommittee membership rosters from recent Congresses in the Congressional Directory, available both in print and on the web. Subcommittee membership rosters have been published in the Congressional Directory since the 93rd Congress (1973-1974). Since the 80th Congress in 1947, the United States Code Congressional and Administrative News, a publication of West Publishing, has included subcommittee membership rosters in its committee lists.
A list of Senate committee and subcommittee assignments is published by the secretary of the Senate and the clerk of the House each congress. This list is republished irregularly during each congress to reflect major changes in committee membership. The title of this list is generally a variation of the title, List of Standing Committees and Subcommittees and Select and Special Committees and Committee Assignments. Lists are available dating from April 1, 1975. The Senate publication is available online from March 2006 to present, as is the current House publication. (pdf)
GPO's govinfo provides links to the Congressional Directory from the 104th Congress (1995-1996) onward. Select the Directory you would like to search and then select "Senate Committees" or "House and Joint Committees." Then select “Standing Committees of the Senate” or “Standing Committees of the House.” Within those documents, members of each committee and subcommittee are listed.
The Congressional Directory may be available in large library systems or college libraries, frequently as a part of their participation in the Federal Depository Library Program (FDLP). Over 1,100 libraries participate in the FDLP, collecting and/or providing public access to government documents. A list of depository libraries is available on the Government Printing Office (GPO) website. Since most depository libraries are within a university or state library, calling ahead to ask about hours is advised.
United States Code Congressional and Administrative News (USCCAN) is a commercially produced publication carried by many law libraries and large public or academic libraries. Check with these types of libraries in your community to see if they subscribe to USCCAN and to verify the policies on use of their facilities.