If you separated them individually, you'd send a separate Excel workbook to each user, with a defined worksheet structure for them to work in. Then you'd get them to send it back to you once complete, save it to a directory and from a master workbook you'd run a macro (VBA) to consolidate them. That's the most secure way that you can do it in Excel, and this is the ideal method if you are going to be emailing the file to your users to complete and then have it emailed back to you.
If you're not worried about your user's hackign your workbook, and you just want it on the network drive that everyone can acces, you could always go with the VBA (macro) route. You'd keep all the data in one workbook (probably different worksheets for each user?) You set up a worksheet that listed a username, password, and the name of the worksheet they could access. After storing the file on a network drive, you'd need a macro so that when any user opened the file they were prompted for their user ID and password, and that would then take them to their sheet.
The latter is probably the easier to set up, but it really depends on how the data will be gathered and your comfort with security vs your user skilset.