Rude. I would always clarify and never assume.
My rule is if I’m going to a party where people are contributing more resources than consuming (eg: a potluck dinner or a house party where people bring a six pack of beer each), then I feel fine inviting friends along without informing the host first. If it’s one where people are consuming more than they’re contributing (eg: Dinner party where the host cooks), then I make sure to ask.
Why do you hate your oldest brother?
Also, I would not bring someone unless I’d cleared it with the host first. Seems like common courtesy
Dietary Restriction guests? They bring their own dishes at first invite (with printed recipes).
That way, when you want them again, you know what you can have to one side.
Uninvited guests/crashers? Haven’t been there since HS.
Host? I’m generous & forgiving.
Guest? I follow hosts rules. If somehow I find myself talked into being someplace where I’m not welcome?
I leave. If there was some drama someone wanted to munch on? Well, all they’ll have to bite is Me. 
If uninvited guests showed up to my house with special “demands” (dietary or otherwise), they would be immediately walked out the door.
“So I invited so-and-so, and they invited you, and you think this allows you to make demands of me? Well, it’s been… interesting, but I think you should be going now. Good day.”
Seems to me that uninvited guests should be as pleasant, gracious and non-demanding as possible, or they should be somewhere else.
“I said GOOD DAY SIR!”
I see Bachelorette Parties, but area bridal showers and baby showers really considered a woman thing? I’ll admit I’ve seen a grand total of three of these things, and those were when I was a child, but they seemed pretty even gender-wise.
Thar you go being redundant agin.