There's a legal authority to compel them, and that's a summons. There's the practical issue of being able to find them, being able to locate them. Occasionally there is great difficulty finding witnesses, as everyone probably has experienced in other committees as well.
Evidence of meeting #16 for Public Accounts in the 39th Parliament, 2nd session. (The original version is on Parliament’s site, as are the minutes.) The winning word was witnesses.
A recording is available from Parliament.