Not at a computer right now. Check out script triggers and file options.
It's in File OPtions and only in FileMaker 12. You can still do this with File Options in older versions, but it's not called a script trigger in older versions.
There's a script step, Set Multi-User [on] that can set the file to be multi-user, but normally this is not done in a script, but rather from Sharing in the File menu.