Is it possible to cross reference multiple Excel Documents?

[SMANNIN]

Essentially, we have one master Excel sheet for forecasting. There are 6 account managers that fill in their incoming and projected work in their own spreadsheets, all in the same format/columns and information. We need the master spreadsheet to gather data from the other excel documents for totals. We're trying to find a way to do this other than, "everyone, fill in this one master document. If someone else is using it or left it open, too bad, you have to wait, try again later," or, "if someone accidentally deletes something and saves the master, oops."