When you capture images of an area and upload them to Mapillary, they are available for everyone to see and add on to with their own images. However, if you are using Mapillary for a private project, you can pay to have the images hosted privately (see our pricing page).
With that said, we encourage anyone to make their imagery publicly available if at all possible. Collaboration is core to Mapillary and by sharing data, everyone wins.
Private hosting is available through Mapillary for Organizations. Create a Mapillary account and set up an organization to get started.