With new young guineas, as with any kind of poultry, it's a smart idea to train them to come when called before letting them out to free-range. It sure saves a lot of frustration & running around, as well as saving the lives of birds. Most poultry like cracked corn, and if you put some in a can, shake it & call "heeere chick-chick-chick" or something, and then give them some corn, you'll soon accustom them to expect a tasty treat whenever they hear that sound.
Another thing to do when beginning to free-range any group of birds is to do it more gradually. You can let just a few birds out, and just before nightfall, to begin. Those few should not go far away from their flock-mates, and want to return to their home by dark. If things go well with those first few, you can gradually increase the number of birds and the length of time you let them out until you can free-range them all day long.