They should probably user their own copy of filemaker to access the database or possibly their web-browser. (Publishing a Filemaker DB to the web has its pros and cons.)
If by Microsoft "works" you really mean Microsoft office, then MS Access and Filemaker could use ODBC to access the same data, but you'd end up creating two sets of every data entry form and report, one set a filemaker file and one set an MS File and that would be a lot of redundant effort to no real gain.
Two of them use the free Works program, not microsoft office. One also wondered if I could convert it to Dbase III. They will use my database to do sorting of their own, so redundancy isn't really an issue.
If the data flows one way from you to them, Filemaker can export the data in a number of formats: microsoft merge, Excel, Csv, text, and DBF to name a few.