Some writers will say they don't use a critique group; others wouldn't consider their manuscript polished without the input from one. There are both online critique groups and face-to-face groups. Whether you use a critique group or not is up to you, but there are ways to get the most out of your critique group.
First, a critique group should be comprised of fellow writers who share common goals, and it works best if they're writing in the same genre. For example, if you write children's picture books, and another person writes memoirs, it fits best if you join a group that writes picture books and let the other find a memoir group. Look at it this way: if you have a forest green couch, and your partner buys a bright orange floral couch, there won't be a good fit in the living room!
Second, a critique group should be comprised of a small group of people, usually no more than five or six, who are committed to each other and helping each other polish their manuscript. Most online groups work that one writer submits a piece to the rest of the group and expects a critique back by the end of the week. Face-to-face groups can work the same way as well, with the piece to be critiqued sent via email a week prior to the next group meeting. Obviously, in either of these situations, it wouldn't work well to have more than five or six people because of the long time between critiques.
Third, when you join a critique group, you have the right to expect honesty in the critique from your fellow group members. Honesty, however, does not equal brutality. Just as there is always writing in the submission that needs addressing, there are good points in that person's writing as well. The good critiquer will point out not only errors, but also the places where the writer did a great job. Brutal honesty has no place in a critique, as it can damage and discourage fellow writers.
Fourth, as a group grows and changes together, relationships develop between the writers, and these relationships can foster a sense of comradeship and security. I have been in groups where the writers have bared their souls to each other in their writing, and did so knowing they wouldn't be slammed. The development of these relationships can extend beyond the critique group, and are an important part of networking with others.
Critique groups can help polish a manuscript and further the career of any writer, often times resulting in relationships that will carry through the rest of the writers lives. Whether or not you join such a group is your decision, but membership in a good critique can be a writer's best friend.