The Editorial Freelancers Association is the largest and oldest US professional association for editors, proofreaders, indexers and related specialists.
There's a wealth of information on their website, some of which will also be of interest to editorial pros from outside the States. One of my favourite resources is the transcripts from the EFA's Twitter Freelance Fridays. If you follow @EFAFreelancers you'll be able to join in with the discussions and see notifications of when the transcripts are published.
Other resources include the training/education catalogue, resource links, society and chapter meeting information, and the EFA searchable membership database.
If you are based outside the US and want to contact your national editorial association, take a look at this international list: Editing & Proofreading Societies.
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I'm an Advanced Professional Member of the UK's national editorial society. Visit the SfEP website for more information.
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