You can join a group by following these steps:
- Navigate to the groups page by clicking on the Groups menu item in the sidebar;
- Search for the group you would like to join and click on it. Make sure Filter on is set to All groups to search for groups you haven't joined yet;
- In order to join the group, click the Join button below the bottom right corner of the cover photo (if group membership is available to everybody in your network) or click the Request membership button (if group membership is restricted);
- Depending on the group type, you're either a member of the group directly (if membership is available to everybody in your network) or your application has been sent to the group owner (if group membership is restricted).