Learn English – “practitioners’ community” or “practitioners community”

Stan Rogers has got the right answer but the wrong reasoning.

practitioners' is correct because what's being refered to is their community, so there's a possessive 's to be added. But because the subject is already plural, the actual s doesn't get written. Nor, incidentally, does it get pronounced in this situation, but pronunciation of the possessive s is independent of whether it's written or not.

practitioner's applies with a single practitioner for anything of his, including his community

practitioners is invalid with no possessive apostrophe, because all you're left with is a plural noun. That can't be used as a modifier to anything, including communities. You can have a dog trainer, where the 'noun' dog modifies trainer. But you can't have a dogs trainer.