Doing creative writing work can be challenging with people who have learning difficulties, however it can also be very rewarding, both in terms of the educational/language-practising benefits and the psychological benefits associated with creating a story - the joy of it and also the confidence-boost of succeeding at making a good story.
I've so far found that mixing art with writing often works well, and breaking activities down into small chunks and, sometimes, writing down to sentence-level structure games. And people don't have to write to do writing: it's great discussing a story as a small group - what will happen next? What do things look, sound, smell like? What should characters be called, wear, carry etc? Why do they do what they do?
Obviously every group is different. I've been working with two people at a care home and we work together to write a story from a stimulus, deciding together what will happen next etc, and one of the two people writes it all down. Great fun!
Other times deciding things at random using dice, or thinking about what a room will look like with the help of an Argos catalogue can work well. Many options.