Here’s what consumers have asked with answers from Grammarly staff and previous consumers.
You can't. The same errors show up again and again and again.
That is a statistical question that can only be answered by the owners of grammarly. But, in my opinion, I would think the bulk of the users are students, collegiate and high school level, and many of the ambitious writers needing a second set of eyes. Personally, I would not recommend this site to anyone. I've based my opinion on a series of tests I conducted in search of inconsistencies within the program's objective--they failed miserably.