How can we resolve our dog's new aggression towards leaving guests?

Our adopted shepherd or akita mix (we're not sure) is about 1 year old, we've had him almost 3 months. Only recently has he begun doing this though. He warms up to guests nicely and is fine while they are visiting, but when the person goes to walk out of the house he begins pursuing them and barking very aggressively. Has anybody dealt with a similar issue or have any ideas of what is behind this? Any help would be appreciated!