Fist thank you for posting a good confession.
Its possible they did think you were asleep and just wanted to fuck. However from the perspective of a couple who's done this a few times, there's a good chance that they enjoy the risk of being caught, knew you may have been awake and enjoy being watched, or hoped you'd join in.
There's a few ways you can play this, you can continue to watch silently and enjoy. You can speak with your friends about what happened, but this might feel awkward. Or you can wait till they do it again and make a move - either by playing with yourself letting them know you're awake or by trying to join in.
Again this is coming from the perspective of a couple who's had sex a few times in front or next to a person who's sleeping. When we first started doing it, like your friends we tried to be quite and keep it under the covers, the risk of getting caught was hot. As we've done it more we became more risky. Feel free to DM me if you'd like to talk or know more.
