It depends on how exactly you want to go about it to be honest.

If you want to do this actively, your best bet is to create a group, or offer to help out at an existing group for clinically obese people. I'd say start at an existing group, it makes things a lot simpler for you at first, whilst you can learn about all of the advertising and care that'll go into creating your own group.

Another idea would be to offer people (without insulting them, which is the diffiocult part!) a gym partner in yourself. That way they'll have somebody to keep them company, you'll keep healthy, and you'll make a new friend. Three for one doesn't seem too bad! :)

Hope it goes well for you, whatever you choose to do.

If a person is diagnosed as "clinically obese", he/she definitely needs professional help. The following steps are suggested.
1) Identify local professional trainers that would offer initial free service
2) Develop with trainers a simple effective plan for weight reduction
3) Use free public facilities such as parks, colleges or libraries
4) Get names and addresses of those diagnosed as "clinically obese"
5) Develop a flyer and mail to these people (followed with phone calls) inviting their participation.
6) Record initial weight of each participant.
7) Execute the weight reduction plan for a month.
8) Record weight again and establish amount of weight reduction.
9) Contact local newspaper reporters or TV stations to publish the event and results, serving as advertisement for the professional trainers to acquire future business, as a compensation for their free service.
10) Repeat Steps 1 - 9 at different towns/areas throughout the country and world.