It depends on where you live, if you want to take part in one of their events. The Urban Sketchers are originally an American organisation and are based in Washington, but there are branches in many countries of the world now including the UK and Australia. So if you aren't based in the US, the first thing you could do is to put 'urban sketchers join' and the name of your country or nearest big city in a search engine and get the details of your nearest branch. Then you could either contact them directly through the website, or apply to join one of the workshops that are held regularly in locations around the world.
If you want to be part of the group generally but are not looking to do anything definite yet like go to a workshop, you can join the Urban Sketchers group on Tumblrr, or just go to their Facebook page for some general information to start off with.