To join other survivors in Sector 7 and explore locations around the Crater together, you have to reach level 100 and complete a special quest. A convoy event where you can create a second character will appear on the global map.

It may also turn out that you've already fulfilled all the conditions without even noticing it. Then in the lower right-hand corner of the global map screen you'll see a large round button that switches the game mode between single-player and Sector 7.