You can ask everyone to write down their opinion to an issue or question then split the class into pairs to discuss it then get the pairs to discuss with another pair next to them, then get the groups of 4 to feed back to the whole group.
This is a very interesting question. In case you are a regular teacher of the class & interacting frequently, the way is to create an environment of trust, and then encouraging the hesitant to participate. You also need to off the class inform the sharp and spontaneous ones to make place for others. i promise you, all theses work like magic and can transform a class!
First we have to create interest in taking seminar. Motivate them to present papers in Symposium. Their communications skills improve and they will involve in Group Discussions without hesitations.