With my guineas I know it might be harder now but if you can start on a schedule with them and try to get them every night at the same time. I've learned that mine get used to the same sound. I have a yell that i said everytime when they were younger letting them know that it is time to eat and they will normally go in without much work from me. Probably when it gets colder will be the best time for them to get trained because they will be more relient on the food you provide, since there are less bugs around for food.
Also in my experience it will get easier with several years of guineas in the same coop because the older ones teach the others the routine.