Hi McSunny, and welcome to PPRuNe.
Dropping parachutists is one of the few jobs you can do legally (unpaid) with only a PPL. The other one is glider towing, if you're interested. I don't know how you get into it; I suspect knowing a bit about skydiving would help, though I don't know that for sure.
To be an FI you need a PPL, then to do the CPL ground exams, then the CPL flying course, then an FI course. You can instruct unpaid with all of the above except the CPL flying, but you'll be VERY unpopular - see another recent thread on this forum. And it's hardly worth it; if you've done all the other stuff anyway, you might as well get a CPL. You'll also need a Class 1 medical.
Don't think I've missed anything out; might have, long week.
Good luck,
Whirly