Alone, you can do everything except the greatest thing. Alone, you can build a simple house but not a palace. The situation is the same with Joomla: you can't do anything marvellous without the help of others. That's where Joomla Groups come in. If you are an experienced Joomla developer or just have intention to put your wet foot in the field, I'm sure there are more you want to investigate about this growing community.