Since the perk of this club is receiving feedback, you have to do your part by leaving good feedback for others. Somethings to include in a comment to make sure your leaving the writer useful feedback include:

A few things you enjoyed about the story. 

Ex. "I like how you described these certain/particular things because of these reasons."

"The characterization of this person was really good/interesting/unique etc because..."

"I thought this part of the story was really good/cute/fun/interesting because of these reasons."

A few things you think could have been better. 

Ex. "The way you described this thing, it was confusing/I couldn't picture it well because.../seemed awkward because..."

"I couldn't really picture/understand why this character did this thing because it didn't seem rational/there was no foreshadowing/it didn't add much to the story/etc."

"I thought this part of the story was kind of weak/nothing really happened/it wasn't very useful to the plot/we didn't learn or see anything new/etc."

Just make sure to include a few things, or at least one thing you did like/that the author did well as well as a few things, or at least one thing you didn't care for/that the author could improve on. That way the author knows that at least they did something right and they feel confident and a bit satisfied with their story, but they also see a flaw from someone else's point