You need to get your island to a 3-star rating, and then talk to Isabelle in Resident Services and ask for an island evaluation. She'll get some feedback from K.K. Slider which will prompt Tom Nook to book a concert. When you log in the next day, you can watch the concert which will trigger a credits scene. (Note: Both of these things have to be done by the "main" player of the island, i.e. the person who started the island.)
The next day you log in, you'll get letter from Harv asking you to go to his island.
Once you land, you'll be told he's AWOL, but the path down the right should now be open.
If you walk down, you'll see Harv and he'll introduce you to the co-op where you can buy new vendors for 100,000 bells. You can only add one new vendor a day.