They cannot actually work on the same document simultaneously. If the
document is on a server to which they all have access, then if you enable
track changes, each user can edit the document (one at a time) and each
users changes will be marked clearly. The document owner (presumably you)
can then do the final edit accepting or rejecting changes as seen fit.
If these are disparate users, then email each their own copy of the document
(with track changes enabled). When each user has made their changes, they
can send email them back to you. You can then open each document again
accepting or rejecting changes and then merge the results all into one final
document.
It is explained quite well in Word Help.
