Start by throwing out an unusual piece of information as you begin to address a group. "Free Lamborghinis for anyone with an "L" in their last name," for example. Work it in with the same speech pattern and sound level as whatever else you're talking about. At least one person will hear it and ask about it. This will begin the discussion about listening.
Explain that everything that is said might remind them of something they want to say, and that they might therefore stop listening and simply wait for a good time to interject. But continuing to listen might lead to their finding that subject covered and expounded on, so that they learn even more about what they first couldn't wait to share.
Say, "Please don't interrupt" whenever someone does. You may have to say it a thousand times, but people must get used to hearing it and heeding it. Go back to those you stopped from interrupting and ask them to now talk about what they wanted to say. Encourage everyone to listen to them.
Ask the person who waited to speak how it felt to then speak and be listened to by everyone.