How do I visit cities of other players?

You can visit the cities from anyone on the World Map, via your Fellowship, or via the Rankings. On the World Map, you can visit by clicking on the city you'd like to visit. Via the Fellowship or Ranking, you can do so by clicking the player's name, and it'll give you an option to visit.

To provide Neighborly Help to someone, you will need to have discovered them on the World Map, or they will need to be in your Fellowship. To provide Neighborly Help, you will have to visit their city, and click the helping hands icon at the bottom of the screen (shortcut key: E), and then select the building you'd like to boost.