Dear Ben,
I wanted to write to say sorry about last weekend, because I know I upset you, and I don't feel good about it.
I should have asked you before inviting my friends to stay over. Two of them came from out of town for a concert and missed the last train, so I let them sleep on the sofa without thinking. It's your home too, and you had every right to be asked, especially with your exam on Monday.
I realise that the noise and the mess in the kitchen when you got back made things worse, and I'm sorry you had to deal with that.
From now on, I promise to ask you first, and I'll always give you at least a day's notice if I want friends to stay. I'll also make sure they leave the place tidy.
Can we talk it over this evening? I'll cook dinner as a thank-you, and we can agree a few house rules together so that nothing like this happens again.
Best wishes,
Alex