How are you trying to share this file amongst more than one user? Hopefully, you are opening the file on the host machine with Filemaker, then each user is using open remote to access the database. Opening FileMaker files directly from a shared directory is hazardous to the integrity of you file so I had to throw that warning in here.
If you find this option greyed out when you open the file on your machine and pull down the File menu, it's possible that you do not have the file open with a full access password. Try opening the file while holding down shift (windows) or Option (Mac) and see if a password dialog appears. If so, it's set up in Document Preferences to open itself with a less than full access password.