I love my writers group. We get together at our library every three weeks. Our leader set up a loop so we can email our submissions to everyone (mailings to those with no email). We usually have 2 or 3 writers submit their work for discussion. This is a time for helpful comments on the writing, not a time to knock down a writer. If a member sees something that could be fixed, she will point it out to the writer, then suggest ways to make it better. The writer has the choice of incorporating the suggestion in the story or not.
I think one of the best rules is knocking when someone agrees with the critiquer especially on an excellent writing section. It is so funny when we are all knocking our knuckles on the table in agreement.
There are so many personalities in a writers group. It makes for a lively discussion and lots of fun. Just remember if you are being critiqued, take it with a grain of salt. You have the final say in how you want to present your story.