What is it that the users are attempting to do?

By default (someone correct me If I'm wrong, as there is no documentation on this xproc), only sysadmins, msdb dbo's (?), and members of the defined msdb SQLAgentRoles (SQLAgentOperatorRole/SQLAgentUserRole/SQLAgentReaderRole) have the ability to use that procedure.

xp_sqlagent_enum_jobs is basically a nice, quick little way to view running jobs.