Leave space between mushrooms, as a packed layer isn't going to cook right. Mushrooms release liquid as they cook, and there needs to be room for it to evaporate.
Improperly sauteed mushrooms can come out of the pan dripping, soggy, and even greasy, and when they're crowded, they turn out wet and mushy since the moisture can't be released.
Also, make sure your pan is up to temperature before adding the mushrooms. They act like little sponges, so failure to heat the pan can cause your mushrooms to suck up the oil.
Crowding the pan can also cause the pan's temperature to drop, essentially creating a double-whammy situation where the mushrooms are both soggy and oily.