Public groups can be joined by anyone. Public groups that you have joined appear in a list on the left hand sidebar under the Groups header. They are denoted by the tag icon. You can search through a list of all public groups by clicking on the Groups header.
This will display public groups you have already joined, and ones that you can still join. To add a public group, click the + icon and select Public for the group type.
You can leave and join a public group any time you wish. For more information on this, see our article on joining and leaving groups.