If you want to share a single file with some or all of your clients, do so from your Library >> Files area by the following steps

1. Find it in your Library listing, and click on the card.

2. In the highlight panel on the right, click the share icon for the file:

3. In the share pop up that appears, click on the "Actually I want to share this with several clients..." link.

4. Select some or all of the clients there, using the shift and/or Ctrl keys as needed.

Note: A client who is greyed out in this display means that client already has that file shared with them.