Co-authorship for grammatical editing
My personal opinion is no, editing at that level is not sufficient. 

One can peruse guidelines for your field. Taking Nature as one fairly broad journal, they state: 


  Each author is expected to have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data; or the creation of new software used in the work; or have drafted the work or substantively revised it


Editing for grammar does not rise to that level for me. And, I can say that over time I have edited a large number of papers for grammar and usage, and never would have asked for authorship. A separate set of eyes looking solely at word usage, spelling, and sentence clarity, while broadly useful, is not at the heart of the work. (Heck, the journal may suggest edits, but would not ask for authorship.)  

Such, shall we say, copy editing is quite different from having deep conversations on how to do something and what it really means. While these sorts of interactions can come up, particularly in the early draft stages, they are not about editing, but about analysis and interpretation, not grammar.