If your know-it-all friend advises you not to say, "Me and John went...," she is giving you grammatical advice. In other words, she's correcting your terrible grammar. (BTW, it should be "John and I...")
The adjective grammatical comes up most often in English classes, since it describes anything having to do with parts of speech, syntax, and other elements of a well-written sentence. The Latin root word is grammaticalis, meaning "of a scholar," which in turn comes from the word grammaticus, "pertaining to grammar." If you're an authority on the English language, you can feel free to describe yourself as a grammatical scholar.
