in my experience, keeping them in longer gives them a better chance to bond with their home and return home
having said that, Guineas do require some training and a lot of patience and some effort in the beginning
we do not let them roost in trees because the owls and raccoons would get them - Guineas are pretty much blind in the dark
here is a great article on how to train them to come home at night:
http://guineas.com/articles/training.php
also, don't be discouraged if yours seem to fly to the trees and refuse to come home - they go through almost a teenager rebellious stage but then relent quickly
a good thing to have is a large fishing net with small netting (got mine at Walmart in the sports/hunting isle) and be super patient and wait until dark and then catch any Guineas that refused to come home - chasing them before dark will likely result in them flying up higher into a tree and out of reach - I did insist that everyone comes home and gets securely locked in the coop at night and now they do it on their own when I ring a cow bell and offer treats (white proso millet works best for Guineas)
oh, yes, and most people are surprised when the Guineas don't come out at all at first, even for days, when you leave the door open - do not be tempted to chase them out - they will come out on their own and you don't want then to get scared in the coop and be afraid to "go back there" - you want them to feel safe at home and come out when they are ready on their own
also be aware that the outside of the door may seem strange to them when they turn around and see it from the outside going in, so they may need some (gentle) encouragement to have them go back in - the more they go in and out in the beginning the better - so they learn that it is safe to go back in - they will explore further and further out soon
good luck!
