Do you have a main  method or the Party constructor?

If you did it would look something like

class Party {
public static void  main(String[] args){
Party p = new Party();
p.buildInvite();
}

public Party(){
}

public void buildInvite(){}
}

Note that you don't actually need a constructor with no arguments, but I
find that it is good form to put them in until your brain positively
knows what is happening.

Also do you have the directory in question in the class  path, or failing
that "." in your classpath (. is the current directory).