In the first instance, the clerk, on behalf of the committee, usually issues an invitation to the person. If the person can't be found or refuses to come and the committee nevertheless wants to proceed, they have the option of summoning the witness to appear. To help bring the person to the committee table, the House of Commons has a bailiff who can assist in locating him or delivering the summons.
On February 14th, 2008. See this statement in context.