Oh gosh, I could write an essay! I felt exactly as you describe for ages but eventually decided to do it and I only experienced encouragement, friendliness and camaraderie. I also knew that Pippa, who is now superhero fit, was not always so, and thus really understands the journey that everyone goes through, I think this makes a big difference.
Also if this person would like a non fit, extremely non skinny person for company, I'll come back and buddy, I so need to do that and it will be a compelling reason to re-join! It's also fun and will get your endorphins jumping up and down!