button mushrooms can be frozen without cooking first, just wipe clean, spread onto a tray and open freeze. Larger mushrooms need to be sliced, then saut'ed in butter for about a minute, then packed in a rigid container with the cooking liquid, leave to cool, then freeze.
I find the texture goes all spongy & horrid if frozen raw
I agree with tambourine - they dry quite nicely but I do not like them much frozen raw or blanched- if you must freeze them I think they are best sauteed
I would simply saute them in a little oil (thickly sliced), then open freeze them.. When totally hard, just pile into a freezer box or bag. They won;t be much use in salads etc (ie raw) but will be ok in stews, casseroles, chillies, curries etc.