An easy way to get each individual to participate is to allocate some sort of task and then have each person report back to the class. Not everyone is comfortable with this, so you need to ensure your training group is supportive and gives every person a fair go. It helps build confidence, tests whether your participants are learning, and ensures every participant contributes.
One of the things I do in my training, particularly in small group work, is to sit with each group for a time during the exercise I have set and monitor each individual's input to the task or discussion. If, for example, I have set a task to discuss an aspect of what we are learning, and I have groups of say 3 or 4, I will watch and see if every person in the group is contributing to the discussion. If I see someone sitting back and letting his team members do all the work, then I intervene to ask for his/her thoughts and bring that person into the discussion. Sometimes, people just need a little helping hand, and as trainers, that is part of our job.