How do I get my housemate to stop bringing people over without telling anyone?

I share a house with 3 other day people, and one of them constantly brings other people into our house without so much as letting us know he’s doing it.
There have been far too many occasions in which i’ve been out doing my grocery shopping and come back to find people i don’t know sitting in our living room. He literally let some guy (who’s name i still don’t know) use our washing machine at 11pm because the guy didn’t want to pay to use the uni accommodation washing machines.
It frustrates me that I can’t comfortably live in the house i pay rent for without worrying that he’s going to bring more people over. i don’t expect him to ask permission, he loves here too he can bring people over, but I fully expect some warning that it’s happening, a text message or something.
How do i bring this up in conversation? because i know he won’t see a problem with it as this was an issue that we had with him last year too, but it fell on deaf ears when we brought it up to him.

You say you don't expect him to ask permission, but you kinda do, as you're asking how to stop them.

To be honest, its a shared mixed sex accommodation, a degree of busy informality and a relative lack of personal privacy is somewhat part of the deal. Most people don't share a home with random unless they kinda have to

It's certainly something you can mention is making you uncomfortable and ask nicely for a warning in future, but I don't think you're entitled to such. I'd see it more you being sensitive rather than him being unreasonable.

I'm wondering why this wasn't sorted before he continued living with you. What house rules did you agree?

see when it happened last year, he had apologised and said that he would stop doing it, so we thought it had been sorted.
it only occurred to us that it had fallen on deaf ears this year when he continued to do it.

i don’t mean stop them from doing it, i mean stop them from just not telling any of us that they’re doing it. i have no problem with the general concept of them bringing other people over, but i have a problem with them not telling anyone that they’re bringing someone over. literally all i want is a heads up so i’m not walking into the house to see some stranger sat on the sofa that i didn’t know was coming