Hello Julia,
I am sorry to hear that your first gathering of transgender folks was so bad. I have been to a few different groups and while some were better or more relevant than others, none of them were that disrespectful or mean spirited. It should have been addressed and dealt with, either by a moderator or just the other folks there. I am not aware of any gathering, transgender or CIS that people would be allowed to be openly disrespectful of others. I am glad to hear that you spoke up to the two women in the manner that you did, it took strength to be that expressive in a strange and uncomfortable environment. Go girl!
As for clubs vs support groups, they have some things in common and some distinct differences. Clubs tend to gather for shared interests. There is a "Club" of transgender women in a city near me that gets together every weekend for the express purpose getting dinner out and then clubbing. They dance until closing time, grab a bite to eat and head home near sunrise. I wish that I still had the stamina for such things. Support groups are usually set up with the purpose of providing a safe environment for people to come and learn, experience, share time with other curious people. There is one just north of where I live that caters to a broad mix of LGBTQ community members that for the most part are new to knowing themselves. A moderator is there to keep some form of sanity to the gathering and protect the environment so that people are not treated the way you experienced at your gathering. I have to believe that had anyone been as disrespectful as you describe it would have been swiftly dealt with. Many times those that attend some of the support groups will join with like minded folks for friendships and/or off site meals and smaller gatherings, perhaps verging on loose knit club concepts.
I have made some close contacts through support group/clubs that have become dear friends, the kind of friends that I rely on to keep me sane and alive.
Anne