you would actually say: "Dies ist nur für uns." but that sounds way too fancy, nobody would say that! but have a look at what I commented in this thread, maybe that helps, too: https://www.duolingo.com/comment/25229053

nur means "only"

einfach means "simply"

Perhaps you're confused because "just" can cover both meanings? (It's just a mouse = It's only a mouse; That's just the way it is = That's simply the way it is.)