The House formally appoints members of select committees, but it’s left mainly to the parties themselves to put forward names. The process for getting appointed to a select committee varies from party to party, although the method should be transparent and democratic. If you want to be on a select committee, your first step should be to consult your whips who can offer more information about your party’s internal processes.
If you’re successful in your party’s internal elections, the whips will give your name to another select committee called the Committee of Selection, which will put it to the House in the form of a motion for approval in the Chamber. While other MPs could object at this stage, this is generally a formality.
The size of select committees varies. Most departmental select committees have 11 members, but some are larger. For example, the Northern Ireland Affairs Committee has 13 and the Committee on Exiting the European Union has 21. Each party is entitled to a number of seats on select committees in proportion to the number of MPs they have in the House. The places each party is awarded aren’t always evenly distributed across the available committees. Negotiations about these allocations are done through the Selection Committee, whose members are mostly whips.