You need to use either FileMaker Pro peer-to-peer sharing or FileMaker Server to host your databases. You can't just upload to an FTP and have people open those files, as they'll conflict with each other.
Note that there are a number of FIleMaker hosting providers, such as ourselves, available to host your databases for you. Please contact me via private message here or at jmay(at)pointinspace.com if you'd like more details on our services.
Using FileSharing to try to share a FileMaker database file can result in a damaged file.