This is easily fixed but will takes some doing and work on your part. Greys are strongly bonded and often make lifelong commitments. The parrot thinks that it is your mate and won't allow anyone to steal the affections of its mate. You now need to teach it that it is in a community situation and has to get along with others, as it would in nature. Non-breeding A-Greys form flocks, this makes it easier to find food and many eyes make for an early predator warning system.
The first thing you must do is keep the cage well below eye-level, and do not allow the parrot out when there are people other than your goodself in the room. Get other people to feed it when you are around, if it ignores others or tries to bite them through the wire, put a cover over it and ignore it. Any screeching is always followed up by removing the cage to an empty room. Never allow the bird on your or anyone's shoulder, this gives it a feeling of being in control.
Do some googling on parrot training. There are many good experienced trainers who are willing to share their knowledge. This is not going to change overnight but is definitely easily fixable with dogmatic and strict training.