I have run a very successful tasting group since 2006 aptly named CLONYC (Cabenet Lovers of New York City).
What started as a tongue in cheek experiment has blossomed into an amazing group of like minded, unsnob-like and friendly set of folks. People joked with me early on and said things like ‘What are you going to do after you run out of ideas for a Cab focused group around dinner number 3’. After 34 dinner/tastings, we have yet to run aground of ideas. We have hosted some of Napa’s biggest names, and even a giant from Australia.
Here is some of what I learned:
I try and keep in fewer than 12 people. 10 is usually preferable, but 12 works ok. I do ALL the pouring. I had a bad experience way back when someone poured 4x the amount of Scarecrow into his glass leaving 3 people with none. Since that I have controlled pours— I am the hardest working man in wine dinners. 
I do these with people I enjoy dining/tasting with. Some have been weeded out in the years for different reasons and some have palate changes which is to be expected, and don’t return. We get a few new members a year, and I am always getting requests to join.
The format that works best is the ‘everyone brings a bottle’ one, usually on theme. I ask for a list and usually choose which would best fit the night’s thematic plan. Sometimes we go blind, whereas I am the only person who knows all the wines.
When we host winemakers/winery owners: I have found these to be some of the most generous persons I have met. (Thomas Brown brought something like 20 wines to the group for tasting). Attendees need not bring a bottle on these nights. The bill is split equally and is larger due to dinner costs for our guests and other things like gift bottles, etc.
I have a home base restaurant where we do all of our dinner/tastings. There is a private room and the staff does a bang-up job. They are well versed in what our needs are and it we area at a point now where it has become almost effortless (other than the pouring).
We tip well. This is key. Our servers get to what amounts to 45-55% at the end of the night (The staff never need touch a bottle of wine either). Working my teen years as a busboy and waiter has taught me enough to know that in the service industry, you are only as good as the service you provide and how much people are willing to reward you for it. I/we are loyal to those who deserve my/our loyalty, and North Square Restaurant in NYC gets that loyalty. Does this mean I never expect problems? Nope, but what then becomes equally as important is how those problems are dealt with. Keep us happy, and we will keep you happy. Everyone wins.
One important item I have found is doing math and collecting monies at the end of the night. Early on we were regulars at Zoe in NYC. (Since closed). As great as a restaurant (and as nice as a people you could ever want to meet), the one grip was a bill that was nowhere near consistent. They would rent glassware when we needed and add to my bill, and sometimes offer aged prime steaks, which confused my math skills once a dozen bottles have been ‘sampled’. Together, the restaurant and I have worked a somewhat consistent billing schedule these days. Sometime it fluctuates if I have special request, but for the most part I can pay the bill without going into my goes-in-tus.
I keep two wine blogs: One for CLONYC and one for my personal notes. I try and write notes for each dinner/tasting and keep it updated as best I can.
Bottom line: Keep it fun and lively. Don’t invite snobby people make sure everyone gets 2.2 ounces and all will be right.