Before you reach Nimbasa City, you must first visit Join Avenue. This place replaces the normal gate that you'd find before a city. Join Avenue starts off empty however and will require effort on your part before it will grow into a proper location.
Once you enter for the first time, you'll be greeted by a suited man surrounded by some girls. The man--Join Avenue's owner--sees the potential in you and puts you in charge of Join Avenue's affairs. He also leaves you with a pair of blue-haired ace advisors, in case you ever need help.
From here on out, you can invite visitors to set up shop at Join Avenue or recommend visitors to check out particular shops. Doing so will increase the rank of Join Avenue and the rank of your shops. Obviously, the higher the rank, the better the rewards you can get.
The shops in Join Avenue offer a range of diverse services, so it's worth dragging in people to set up shop so you can use the shops yourself. To check if there are visitors, you can open the Town Map and hover over Join Avenue to see the numbers.
Anyway, after you're done, it's time to head forth to Nimbasa City!