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Translators' Academy (TA) has a sticky thread called Request a Full Proofreading where translators can post proofreading requests. Many established translators already have proofreaders, but there are many who work on their own. Feel free to introduce yourself in the thread and tell them that you could help. Make sure you mention that you are a native English speaker. American English and British English are quite different, so you probably want to mention that, too. Many proofreaders don't speak foreign languages, so you don't have to worry about languages other than English.
A good proofreader would know how to correct spellings, grammar and the speech style used in tl. If you are unsure of the difference between American and British English, read wiki [link]. Remember that manga tl is colloquial in most part. One thing you want to keep in mind is the language structual difference between English and Japanese. Since Japanese uses passive voice to show politeness, beginner translators may use the same voice patterns in English. You may want to change the phrase to active voice to make it sound more natural.
If you are interested in joining a scanlation group, Staff and Project Resources would be a good place to visit.