Levente, are you 100% certain about the ID of these mushrooms? The reason I ask is because Agaricus is a very big genus and the cultivated mushroom (A. bisporus) has several wild look-a-likes that can be confusing for even the pros. Also, where do you live? For example, wild A. bisporus is common in coastal central California, but rare in the Pacific NW. Agaricus campestris, the Meadow Mushroom, is the species most commonly confused with wild A. bisporus.
At any rate, if you want more Agaricus, then I would recommend that you make spore emulsion and begin dumping it all around your property, especially grassy areas around
compost piles. To make spore emulsion, throw a bunch of old mushroom caps into a
bucket, add a spoonful of molasses and a pinch of salt. Then fill up the bucket with warm, non-chlorinated
water. Stir it up, let it sit for a day or so and then dump it or
spray it all around your place. I would also spread of a lot of lime around your place too, since Agaricus mushrooms LOVE calcium. And finally, if you don't have a sprinkler system, I would recommend getting one since Agaricus will really benefit from late summer
watering. Running a mister on established patches can increase production, too. Do not let your grass turn brown or dry out. Good luck!
PS: Complete instructions for growing Agaricus and making spore emulsion can be found in the
books by
Paul Stamets. Growing kits can be purchased from Mushroom Adventures and used as spawn, too.