I think it really depends on what the participants are interested in and what kinds of cellars they have.
My favorite wine dinners are tasting menus with a wide variety of wines, with no theme other than terrific, fully mature wines. I tend to do these at a local restaurant that offers a tasting menu anyway and has shown a willingness to work with us to customize one for our wines. An example here would be one bubbly, one or two whites, four or five reds, and a dessert wine - 8 wines total with an 8-course dinner, for 6-8 people. Again, no theme, so the reds could be one Burg, one Brunello, one Rioja, and one Napa Cab, or whatever - with the dishes (small portions, obviously) chosen to match the wines. You can have each person provide one bottle (coordinating so you have the right number of bubbles, white, red, and dessert) or take turns letting the “host” bring them all. These are great for groups of experienced drinkers with deep mature cellars - and they can be great for tasting wines the members aren’t that familiar with if you follow the “everyone brings a bottle” approach and your group has folks with different cellar foci.
I’ve done plenty of others where the theme is pretty narrow and the purpose is more comparison. So for example one theme could be a check in on 2010 Bordeaux and you could do flights by commune or the like. These again are better for more hardcore geeks who would be interested in things like how the right bank 2010s are doing compared to the left bank 2010s.
I’ve done others where it’s a much looser theme and everyone brings a bottle. “Wines you like with BBQ,” “Pinot Noir from anywhere,” “wines from unusual varieties,” etc. These are better when there are more beginners in your group or you just want a more casual approach.
If you’re really looking for more education and have a lot of beginner members who, maybe, have never had an Italian wine or a chardonnay wine from anywhere other than California, etc., and don’t have deep cellars, I’d suggest (though I haven’t done this one) more formal themes focused on relatively inexpensive and newly-released wines. Members could suggest various topics they are interested in learning about and you could rotate host duties. So one might be Italian reds and the host would bring several representative wines from the major regions of Italy, another could be Cabernet-based wines from around the world, etc. This would be a great way to approach it, IMO, for maybe a dozen tastings over 2-4 years, at which point the group may want to ditch the “wine 101” format and move to something more serious. Once folks have developed a sense for what they like and once they’ve built up cellars, they’ll be less interested in these “intro” tastings but for newbies I think they’d be invaluable.
Of course, you can also just rotate hosts and let the host do whatever theme or lack of theme the host wants to do, with the host providing all the wines for that evening. The host with the varied mature cellar could throw a dinner like the tasting menu I suggested first. The host with the deep Bordeaux cellar but little else could throw a dinner like the “2010 Bdx in flights by commune” I suggested second, etc. That allows each host to play to the strengths of his or her cellar.