If other users require access to a file, the uploader must share the file with them. This can be done either at the time of upload or later on once the file exists in the system.
The process for uploading a file is the same regardless of whether it is to the Company Files page or a user's profile, but you can see how to do this with steps on how to share the file with other users upon upload here (for company files) or here (for profile uploads).
To share a file after it has been uploaded, you will need to navigate to the area it was uploaded to (either the Company Files page of a user's profile).
Find the file and hover over it, then click on the cog that appears to the right of the page.
You can then choose who you wish to share the file with. If there are already users with access to the file, you can choose to remove them by clicking the 'x' next to their name.