Attas look like Grey aliens of real life; they have large black eyes, very small nose, a large head and grey skin, which is where they get their name. They look like typical aliens of popular culture and Ufology.
The Attas at some point in history encountered the Progenitors, and from Hidden Messages implies, destroyed their homeworld of Nibiru forcing the Progenitors to flee to Earth. The Attas seem to have followed the Progenitors and likely raided Earth, possibly hunting down Progenitors and messing with Humans, though it's unclear.
The Attas during the events of Conduit 2 sent a spy spaceship to moniter the events going on Earth. This attracted John Adams attention who had plantary defenses to shoot it down. It landed in Oxford, where The Trust began to collect the leftovers, albiet being slowed by Michael Ford.
A dead Attas can be found in the crashed Attas vessel in Oxford.
The Attas can first be seen in The Conduit in a picture during the intro, however since Attas are referred to as "Greys" or "Grays," you don't realize they one in the same. It isn't until Conduit 2 that Greys are revealed to be Attas.