Explain the main idea

Ask people to write a couple of sentences in answer to this question on a sticky note:

Gather everyone’s notes to a board where everyone can see them and ask people to read them all before they go.

Benefits

By re-stating the main idea in their own words, they are more likely to remember it. Reading other people’s ideas will give them new perspectives. It also helps you judge whether they understood what the main idea was supposed to be.