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.