One of my friends in my NaNo group (a writing group, but not a crit group, although two of them are betaing my stuff) was part of a very close-knit crit group. They were all friends, they went to Italy together. They had very specific rules for critting that sounded fabulous to me, but the point to this story is that they considered themselves close friends.
This story is not about how critting tore them apart, although it's easy to see how that could happen.
No, one of them gave my friend a printed-out copy of her novel to read for fun. She did so right before April, when my friend was going to be doing Scriptfrenzy, so she was like, I'll probably read it in May, but I look forward to it. But she kinda got the vibe that the other girl was hoping she'd read it sooner, so she was reading it each morning as she got ready for work.
One day, she spilled an entire cup of tea over the manuscript. She was running late, and so she grabbed some towels and threw them over the ms, hoping that they would sop up the tea. No, instead she ended up with a brick of paper that she could not pry apart. It was completely unsalvageable. So she threw it away and worried about how to tell her friend.
The next day the friend called. She had gone to a writer's workshop a bit ago, and completely changed her novel based off comments she got there, but she hadn't saved the original copy, and she decided she wanted to go back. My friend had the ONLY copy. When she told her what had happened, the other girl flipped out, then told everyone else in the group that my friend had stolen her novel to try to publish it herself, and they all turned against her.
I don't know what the moral of the story is, except "back up your work."
Anyway, there are definitely times that writers can get jealous and try to sabotage other writers in their groups, but there are also writer's groups where everyone is supportive, but can also be constructively critical too. My NaNo group is mostly oriented towards the beginning stages of the novel, "how can I plot this?" "how many words did you write today?" "did you work on that novel of yours I love this week?" type of stuff. But they're good for betaing too. I'm also part of another close writing group that started here on AW, and we often beta each other, and know how to be critical for full novels, excerpts, and queries, but also how to hold hands and support each other when it's needed. If I know a novel's not ready to be sent out, I will speak my mind about that, but if the writer sends it out anyway, I'll still be there cheerleading their queries and fulls.
It's just a matter of finding those groups.