I think the attire should be mentioned on the invite (Cocktail Attire Requested
), and the invite itself (even if an evite) should look more formal.
But also, I think the hostess and her closest friends should spread the word in conversation pre-party.
And I'll tell you, from my experience, my social crowd did this too - we did many years of chips in plastic bowls, salsa straight from the jar and drinks in red Solo cups and then started with some cocktail parties - the guys tend to still dress down, its only really the females who do the dressing up bit. the guys will dress up a little - jeans with a button up shirt and leather shoes instead of sneakers, but no sports coats certainly not suits (well a few slick dressers will wear suits, but they are the guys who always dressed up more then the others). YMMV of course, and its fun anyway, just putting my experience out there.