You have received a shared document and want to add a document so that the creator of the share can see it. Here's how:

Step 1: Double-click the share name to open it and view the list of documents.

Step 2: Click the icon at the top right to add a document from your computer.

Step 3: Select the document and click "Open".

If you want to upload more than one document, hold down the CTRL key and select all the documents you want to upload.