Which strategy works best to people feel more comfortable in meetings?

Your "encourage everyone to listen attentively".

As you are to do this, this would then make the person feel great, as in comfortable, and welcomed. You would also want for them to feel welcomed, make sure that you would speak with good manner, and also, as you are to do this, these people may even attend future meetings without a invitation, and therefore, this would then allow for them to feel free in the meetings they would be attending.

Humor would make everyone feel more comfortable. It would help relax everyone.