Group plantings are a great way to create landscapes in miniature and there were some excellent examples of group plantings at the Wessex Bonsai Open Competition to choose from and inspire you to perhaps put a group together.
Building a group is not that easy, it requires the appropriate material to start with plus the all important idea of what you want to try to create in order to bring the image to life. Scale is important as well as the overall finished impression and trying to balance all these factors is not as simple as it sounds. Often it is good to start with immature trees so the group grows up together and over time can mature but this all depends on the tree species and type of group being created. There is a good article by Harry Harrington on the basics of developing a good group on bonsai4me.