Good to hear from you, Jim, and thank you for your interest! I myself am a DTM
Feel free to join the Google Group:
https://groups.google.com/forum/#!forum/toastmers that's all that is involved in joining the SIG. It's really just a discussion board/list for people to discuss starting Toastmasters clubs catering to Mensans. There's been little activity yet. I just hope it will be useful for people to share experiences and advice... and I am curious to learn how many Toastmasters clubs there are that are started by Mensans! Since we can't use the word "Mensa" in club names, there's no easy way to tell, but I know there must be several.
I've been working on starting a Toastmaster club for Mensans here in San Antonio. We made it an open community club, to attract more people and in the hope of helping other people learn about Mensa. It's been slow going to get enough interest though. I tried inventing a few new roles to appeal specifically to Mensans :)