I've been doing support groups of one kind or another for almost 10 years now. Here's a few things I've learned in that time....
Find a free meeting site. Here's some places to look for that:
community organizations such as the Lion's Club, Moose Club, Masonic Lodges, etc.
local fire departments
schools (public or private) as well as school board offices (ask about the room where they hold school board meetings!)
after school program sites
churches (even if it isn't YOUR church!)
libraries--including school libraries
hospitals and home health care agencies
Dr's offices--they often have conference rooms or rooms they use for public presentations
some restaurants have meeting rooms, but often expect everyone to at least order coffee
some local shopping malls have meeting rooms available
We meet in the multipurpose room at our hospital. It is free and is set up with tables and chairs to seat probably 50 people very easily. We can use just one side if we like to make the group closer to each other...
THEN, contact all local newspapers and ask if they print meeting notices for free for local groups. If so, get an email address that you can send the notices to for inclusion in the paper. This way you can send your monthly meeting notices to all papers with just one email message!
Try to pick a day that won't change--2nd Tuesday, 3rd Thursday, etc. That way even though the actual DATE changes, the day never does. Folks will remember easier that way.
Make up a flyer about the new group starting up and post it on area bulletin boards, at gas stations, post offices, etc--anywhere they'll let you put one up. Put your phone number AND email on there if you can. Some folks will be shy about inquiring and may be more comfy sending you an email than calling and speaking to you. Consider making the little tear off pieces across the bottom so folks walk away with the number without having to dig out a pen and paper.
Notify ALL area doctors and hospitals. Don't forget about orthopedists and such who also see overweight patients. Also include the psychologists/psychiatrists. Just about ANY doctor sees overweight patients some of the time! Provide a copy of your flier that they can copy and give to interested patients.
Keep fliers in your car! You never know when you'll be out and find a new bulletin board OR meet someone who is interested in attending! Be prepared!