I am going to add 3 new adult guineas to my current group of 5 guineas. Will the 5 guineas "train" the new guineas or will I need to keep the 3 new ones separated and caged for a month?
My recommendation is to pen the new guineas in sight of the your current guineas. Allow the new to become familiar with the old group and go from there. Watch for signs of aggression from your guineas. Once the aggression stops, let one of the new ones in with the current group while being prepared to quickly remove it if necessary. It is likely that your group of 5 will shun the group of 3 unless your group of 5 are mostly one sex and the group of 3 is of the opposite sex. It will be fairly simple to add 3 hens to a group of 5 males or a group of 3 males to a group of 5 hens. Good luck.
I agree with R2elk. From my personal experience, adding a new member to the flock can be very distressing for you and your new guinea. When a new guinea comes into the flock all the others will immediately chase it down and peck its feathers out. They are very aggressive. They will also keep the new member of the flock from their food. This will happen for a few days until they get settled. However the older flock members will still occasionally bully the new member for much longer. Good luck!