sharing a caldendar between employees

Hey Guys,

Three people in my company have asked me if there is a way they can share a calendar between the three of them so one person can make an appointment for someone else. I suggested they email meeting requests but I suppose they want something like the build in calendar in outlook.

Environment: exchange 2007
Vista / XP workstation
MS Outlook 2007

Would I need third party software here? I really dont want additional software installed.

Any help will be appreciated.