On the Stormlight Archive subreddit, there is an Ask Me (Almost) Anything with all the beta readers for Oathbringer. These are readers who read an earlier draft of the book and gave extensive feedback to Dragonsteel and Brandon about it. There's tons of spoilers in the thread, but if you want to know about beta readers, the process, or anything, go ask there! Beta readers have a special flair for this event, so you can easily identify who they are.
Now, we say ask "Almost" Anything because some things we might need to run by Dragonsteel before we can answer it.