To join other survivors in Sector 7 and together explore locations around the crater, you have to reach level 100 and complete a special quest.  
A special convoy event will appear on the global map, where you can create a second character. But it may turn out that you've already fulfilled all the conditions without even noticing it. Then in the lower right corner of the global map screen you'll see a large round button that switches the game mode between single-player and the Sector.