The Senate uses committees (and their subcommittees) for a variety of purposes, including the review of bills and the oversight of the executive branch. Formally, the whole Senate appoints committee members. In practice, however, the choice of members is made by the political parties. Generally, each party honors the preferences of individual senators, giving priority based on seniority. Each party is allocated seats on committees in proportion to its overall strength.
de facto subgroups
Agencies such as the Broadcasting Board of Governors, while claimed to be "independent, autonomous entitities" is nevertheless headed by a board appointed by the President of the United States and confirmed by the U.S. Senate. The ninth member ex officio is the Secretary of State.