The first writer's group I joined was one focused on history/memoir type books. At the time, I wasn't interested in those subjects, and quickly found myself isolated. The older members weren't shy in dispensing their 'wisdom' to an 'inexperienced' writer like me. We sat in a circle with our half-baked manuscripts and proceeded to rip them to shreds and 'correct' the flaws. I returned to my computer feeling like my soul had been ripped out and stuffed back inside, bleeding. The last meeting I attended involved one blowhard dominating the conversation. He went on and on about his philosophy of writing and how his background made him eminently qualified to do what he did.
No one had the courage to stand up to him and tell him to shut up. Until I (literally) stood up and left the meeting.
Needless to say, this horrible experience made me wary of writer's groups in general. At the time, I thought all of them functioned this way. I wasn't going to put myself through that. Never again would I subject myself to such humiliation, I told myself. I had more pride in my work than that. I'd write for myself and no one would ever read a word. Manuscripts ended up in the drawer, or hibernating in the computer's hard drive.
It took me years to get over that. Once bitten, twice shy. I realized that the first group I joined wasn't how a functional writer's group worked. Put a bunch of writers in a room and some of us will get along, and some will clash with each other. Set some ground rules from the very beginning. Is there a particular style of genre that's preferred, or is it a free-for-all? Poetry? Historical fiction? Romance? Assign a group leader for each meeting who keeps discussions civil. Set a time limit on presentation and on criticism from the other writers, so no one author dominates the time, either on their own work or commenting on someone else's.
On-line groups had their own share of positives and negatives. More people could read and comment on your work, but anonymity was a double edged sword. Here's a review on one of my stories (bold text is mine)
"All I have to say is that if these are the results of 'challenging your creativity and gently tearing you to pieces for any failed attempts at improving' I proudly admit it and will cheerfuly continue as long as you decide you want me to do it. :-p Because really this is truly wonderful work. Then again I don't think I would have spent so much time ripping you to shreds if I didn't believe you could do it. "
Um...back-handed compliment? 'Gently tearing to pieces' and 'ripping you to shreds'? What?
Obviously some of the ground rules of common courtesy weren't held in high regard.
Eventually, I did find the confidence to show my writing to the world. I'm currently in a couple of on-line groups. They are both encouraging and exacting. If there's an issue with your draft, they won't hesitate to point it out, but they also acknowledge the gold within the dross. (Kudos to the ladies of The Delphic Expanse!) And unlike the comment above, they don't proudly brag about 'gently tearing you pieces for any failed attempts at improving'.'
Writer's groups can help develop your writing, as far as plot and grammar goes. But you must be discerning and find one that is a good fit for you and the genre you're writing.