Ask people directly. If you know that someone has something interesting that they have been working on, or they have certain or special knowledge that they can share, ask them to give a presentation about it. If someone recently made a presentation at a conference, ask them to present it to the group. (Groups are also great practice for conferences.) Many people who will never volunteer to speak will do so with a little gentle nudge.
You can also connect external speakers. If someone who lives nearby (or can travel in this area) gave a conference presentation, published an interesting article, worked on some cool system, etc., Ask them to come and introduce. Offer them a free lunch for their troubles.
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