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This is gramatically incorrect. It should be You can etc. etc.
May does not apply here because this is a case of can, and can not, as opposed to may, and may not.
For example, unregisters members are unable to do any of these things. Therefore, no matter how hard they try, they cannot post, make threads, etc. However, once they register they have access to these things and can do them. The only way they could no longer be allowed to do these things is if, once again, they no longer had the ability to do them. Therefore, may is inapropriate, and should be changed to "can" immediately.