To tell my friends I kind had to explain why I took my enzymes cuz that was when they noticed. So I'd explain that I had a messed up digestive system and had to take them.
I would usually explain one on one because its easier and then they can ask questions and you can answer rather than having a group of people freak out and stuff. Yeah I know that wasn't very helpful but its what I did.
I used to have IVs in my hand so I would have to explain why I had a bandage round my hand or arm or whatever, but only to my friends. To other people I just said I hurt my hand!!
Hope everything goes ok, if they're good friends, they'll understand and wont freak out or anything (except when you show them freaky scars! Which I don't mind doing and usually show them to freak them out any way, although this is after they know about my CF.)