The Group feature allows you to control who can read and rate your ideas. You can create two types of Groups: Public and Private.
A public group is visible to everyone. By default, an idea posted to a public group can be viewed by anyone. However, you can set the visibility to 'private' when you post an idea to a public group. In that case, only group members will be able to read your idea. One benefit of creating a public group is that it makes it easy for other people to join your group since they can see your group and its description. Public groups are useful when you want to make most of your ideas public, but want to keep some private.
If you want to join a public group created by someone else, you can submit a request by using a link on that group's home page.
A private group is completely hidden from non-members. They would not even know that it exists. A private group is visible to only its members. A non-member cannot see a private group's name, description, its members or ideas posted by its members.
By default, an idea posted to a private group can be viewed only by its members. However, if you are a member of a private group, but want to allow any registered user to rate your idea, you can set the Idea visibility to 'public' when you post your idea.
Since private groups are invisible to non-members, they cannot initiate a request to join a private group. A private group's owner has to add members to his/her group.