These are the salient points cited from the Development Guide.
…key differences between the runtime application and
FileMaker Pro include the following:
FileMaker Pro File Sharing, serving a database on the web, or
communicating with a Java applet requires FileMaker Pro or
FileMaker Pro Advanced. You can, however, use a compatible version
of FileMaker Server to serve runtime solution files.
Runtime applications cannot be shared over a network.
Runtime applications don’t support external SQL data sources.
Since you plan on 6 users, you will probably need FileMaker Server.