Update - we released our juvenile birds into the flock several weeks ago. They seemed shy and uncomfortable with the adults, but did all right, and opted to sleep on a high perch outside of the bird house rather than go in. We were concerned - they would stay there in cold weather and rain, and one even died after a rainy, cold night (possibly due to exposure). Nothing we did would entice them into that house or any other kind of shelter.
Flash forward to three days ago. We decided to remove our poultry netting and let our birds free range for a while. The young ones followed the flock all day, and when I went to close the chicken house that night, there they were! The first night they slept on the floor, but last night they found the perches. So all is well in guinea land. Now we just need to repeat the whole process with our 8-week old birds, when they get a little bigger. But we have a few weeks to wait.