You need to:
1. mention their name
2. thank them for coming along to your meeting / gathering
3. mention 1 or 2 points that you got from their talk
4. ask the audience to give a round of applause.
I would like to thank Jo Smith for coming along to our regular meeting tonight to talk to us about [his/her topic]. This [topic] is a very important one to our group because [your reason for asking them].
I was particularly interested in Jo Smith's talk because I learned / found out from it that [point1] and [point 2] are crucial in understanding [topic] and so I have personally benefited from listening to this talk / presentation.
I am sure that all of our members have also benefited from this talk tonight, just as I have and I would ask you all to show our appreciation to Jo Smith for coming here tonight by giving [him/her] a round of applause.