Group calendaring and scheduling is a tough problem - and there are a lot of solutions. It sounds like you work is a fairly big office, given that there are 7 assistants coordinating schedules. We also wrote about this topic in last April's Meeting Advisor column as well, so be sure to check that out. We mentioned this last April, but it recently got some good press again. This is a powerful group scheduler that got a great review from CNET two weeks ago.
Notes is a combined email, calendaring, database system that's quite mature.
Their unfree version, eProject Enterprise, adds more industrial-strength project management features.