I would say the first is the correct one (with apostrophe), but I see the other one much more.
Don't know if it is relevant, but I'm a British English user.
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.
I haven't seen this specific phrase, so not sure if context would alter the question. Grammatically both can be correct. You can have a community of practitioners that they themselves do not own, in which case the second choice would be correct.
The ess-apostrophe version is correct if it is equivalent to community of practitioners or community to which the practitioners belong (and it's difficult for me to imagine how it wouldn't be with a plural practitioners unless the host sentence is very convoluted).
If it is equivalent to the community to which the practitioner belongs, then it would be practitioner's community.
I've tried to imagine a sentence where there would be no apostrophe, but everything I can think of would require a comma between practitioners and community.
Context is relevant. "practitioners community" means a community of practitioners, whereas "practitioners' community" means a specific community to which the practitioners belong, implying there may be multiple communities or a larger community beyond the one being discussed. The sentence structure and your intent should guide you to the correct usage.
I disagree with @FumbleFingers about the "dogs trainer". Just because it isn't contemporary language doesn't mean it is invalid. A "dog trainer" may work with a single dog at a time while a "dogs trainer" will work with a group. "dogs' trainer" would refer to a solitary trainer that worked with a specific group of dogs. Makes sense to me.
Both forms are grammatically correct. The difference between them is that practitioners' is a possessive noun and practitioners is a plural noun modifier. I found this exchange on plural noun modifiers that has some interesting quotes from Quirk et al., A Comprehensive Grammar of the English Language, Longman.
In short, the doubt about the validity of form #2 comes from a rule in the past that noun modifiers had to be singular (apple tree, vegetable soup, rubber chicken) but today this is not an absolute and there are many examples of plural noun modifiers in everyday use, for example, parts departments, schools superintendents and options markets.
Quirk lists the following situations where a plural modifier may be used:
the singular form might lead to ambiguity
an Arts degree (a degree in the humanities) as opposed to an art degree (a degree in fine art)
there is no singular form of a noun (in pluralia tantum)
a customs officer
there is a need to denote variety
a soft drinks manufacturer [but] a car manufacturer
a topical issue comes forth, often in newspaper stories. Quirk cites examples of Watergate reporting from newspapers:
the tapes issue
the tapes compromise
the Watergate tapes affair
the White House tapes mystery and other examples, including jobs cut.