The Support Group that could save my life.

There's a Trichotillomania (a hair-pulling disorder,I have another journal entry elaborating on it) Support Group meeting this Sunday.Sounds great,doesn't it? It does.But there's one problem: I don't know how I'm going to tell my parents I want to go.
Why would I want to go in the first place? Because I feel so alone having this disorder.I want to meet other people like me out there.I want to have a friend that will understand what I'm going through.
The only obstacle:My parents.I don't know how I'm going to tell them that I actually want to go to a Support Group.

Here's where you come in.How do you usually tell your parents about serious things?
I'll give you a scenario if you need one~
Your doctor called you in today.She said that you have a mental disorder that causes you to eat inedible things (AKA Pica disease),and you're going to die in five months due to everything that you've eaten that can't be digested.
But there is a solution.Inexpensive,but extremely invasive,they can do a certain type of surgery that involves cutting open the stomach and intestines,and removing the items that you've "eaten".
However,you've never told your parents about your disorder,and you need to tell them so they can get you the surgery you need.
How do you tell them?

Comments are greatly appreciated.