When you are joined to a location or city you can:
When you are logged in browse to a state and then a city or location.
At the top of every location / city page is a Join City OR Post button.
If you see the Join City button, this means you are not joined to this location. Click it and you will then be joined to that city / location.
Once you have joined a city and are logged in:
Also at the top of every location / city page is a Nearby button. Clicking this button will allow users to view nearby locations and join them more easily.