Unfortunately, Guinea birds LOVE to roost as high as they can. They are excellent and precise flyers both going to roost and coming down. I too have a open barn (I keep the mini stud horse in it) and the doors are open year round except VERY cold winter storms. The Guinea roost in the rafters which are about 10 feet high. I do keep poultry and game bird in an enclosed area that the stud can not get into. I have an opening just large enough for the birds and they seem to figure out how to get out of this area.
I keep a 60 watt curly light on for them at night. They can see what is going on below them. I did have a few this summer roost in the tree just outside the gates enclosing the stud pin, but they are all roosting in the barn now that the leaves have fallen.
I think a coop just does not do it for a guinea bird. I would however try to put a light in your coop and a treat in it .
I noticed that one night the light was off and some of the birds did not roost in the barn. They took to the tree or on the gates.
Now that it is getting dark early, I just leave the light on 24 a day.