To "say" something means to utter words to convey information, or a meaning. For example, "my mother says I should study harder so that I do well in school". This is a literal example, but you can use the word "says" in other contexts as well. If you look at a piece of art, doesn't it "say" something to you? In other words, all kinds of things convey information and meaning, even if we don't utter words. "nothing says casual like..." In this context the person is being sarcastic. Inviting a hundred people over is not "casual". It is a big event, and the big event "says" that the person thinks it's very important thing. But, the person is trying to pretend that it isn't a big thing. However, even though he says it isn't a big thing, the event itself says something different.

'nothing says casual like ...' = 'nothing demonstrates/shows being relaxed or being casual like ...' Similar structures ... Nothing says stupid like jumping into a rock pool without checking the depth = Nothing demonstrates/indicates/shows stupidity like jumping into a rock pool ...

Nothing says refreshment on a hot day like a tall glass of iced tea. Here says means equals or is as refreshing.

So, like the other answers are saying, "say" means "speak", in its strictest sense, but more generally means displaying information, whatever form it takes. It might also be helpful to know that "casual" could be in quotes, in the example you gave, and it would have the same meaning. Let me know if there's anything I can clear up, or if you've gotten way more answers to this than you wanted.

"Nothing says casual like inviting a hundred people over to mack on one girl." -------- Phrase Pattern: "Nothing says [idea]" Means: "The best way to express [idea]" NOTE: In this context it is sarcastic (挖苦) ------- "says" or "tells" often times can mean to "express" or "show". Example: - "How you dress tells people how much self-esteem you have" Phrase Type: Sarcasm (挖苦)