I think it’s OK but this expression: 誰か is not correct, we usually say ‘ここにある傘は(誰もが)使っていいことになっている’ = All you guys can use the umbrella here. The word ‘’いい’’ means permission. The words 誰か and 誰もが have different meaning. I’m native Japanese, so I cannot describe strictly, I’m sorry ;)

>I learned that 〜ことになっている means to be scheduled to do something or to be expected to. That's an explanation for beginners, which is at first easy to understand but won't get you anywhere in the end, like now. I recommend you should stop interpreting Japanse using English conceptions to understand it by its own logic; They are, after all, totally different languages. *** こと： A temporary conceptual presence. thing temporarily defined as true/good thing defined as true/good by somebody. *** もの： A perpetual/unmovable presence. thing perpetually defined as true/good thing defined as true/good from the beginning of the human world. ここにある傘は誰でも使ってもいいことになっています。 You can use umbrellas here; (Someone made it a rule). 家族というのはいいものだ。 Family is good. (No doubt)