If you've left a shared folder, you can add it back to your Dropbox account on dropbox.com:
Add a shared folder you left
- Sign in to dropbox.com.
- Click Files in the sidebar.
- Click Sharing in the sidebar.
- In the Folders you can add to your Dropbox section, find the folder you left.
- Click … (ellipsis) beside the folder name.
- Click Add.
Dropbox will automatically download the shared folder again, even if a previous version of it still exists on your computer. If the previous version is in the same location, Dropbox will download the new version of the shared folder and add (1) to the end of the name. This prevents an issue where two folders have the same name.
If you'd like to change or restore the previous name, you can rename it to whatever you'd like.