Facebook requires Facebook Platform to join Facebook Groups. What do I do if the author requires me to join a Facebook Group but I don't want Platforms?
It's up to the author to decide how they want to proceed. Some will opt to not let you in if you're not in the Facebook Group, while others will work out a way to communicate with you outside of the group. Please try to be understanding if an author insists you join the group. Running a Reader Team is a time-intensive project and having Facebook Groups may be a key way for them to juggle the workload.