How many Guineas did you get, and how many birds total are in the coop? Guineas don't get along with birds they do not know... they need gradual integration after they get used to each other (housing them side by side, separated by wire for several weeks until you see no aggression). And Guineas do not do well being over crowded, so you'll want to make sure they have plenty of room.
Whenever you bring new poultry into an existing flock you should always quarantine the new birds (keep them away from your existing flock) for 30 days at least, to make sure they are not carrying any diseases or parasites that can infect your other birds.