We're not considering closed groups here. Those also have a role to play; they become intimate as the members grow together. This is about open groups, no membership required.
There are several things you should be aware of when participating. Because you approach the group for a stated purpose, to receive critical assistance, don't bring a poem which you don't believe you should change, that you think is so good that you just want to show what you can do. Your friends and peers may well cut it to ribbons to show you how they would improve it. If you don't think a poem needs improvement, don't bring it. The group is not there to heap praise; don't expect it.
Also consider this. Since it is not a closed group but open to anyone expressing an interest, making a presentation to such an audience can be considered a form of publication, especially if hard copies of your work are distributed. You have very little control over what happens to those copies in the hands of your "public."
Just a few things to consider. Remember, when you ask a friend to become a critic he also becomes an antagonist. Usually the frienship remains unharmed.