I agree that drying works well for mushrooms, but if you do want to freeze them, without taking up much freezer space, that mushroom duxelles is the way to go. A large amount of mushrooms can be concentrated and frozen in small amounts, even ice cube tray size, ready for use in sauces, pasta, etc. I had access to a meadow that had field mushrooms every year at my last home in Somerset, the first time I've ever had more mushrooms that I knew what to do with
! More info and the recipe is on my blog
The Cottage Garden Farmer