Agrocybe parasitica

This is a large toadstool - I have seen them with caps greater than a handspan in diameter. They always appear in groups on the trunks of both natives and introduced species. I have seen them growing 10 - 15m up trunks of poplars on an orchard.
They are easy to identify because of the dark brown "skirt" on the stalk. This was originally a membrane which covered the gills before the cap was fully opened.

These are edible.