If the content is already uploaded, look for the Edit tab in the upper right hang corner. If you have permission to edit the content, the corner will look like this:

Select Edit. You will find yourself on the sharing screen.

If you are uploading content for the first time, look on the right of the page for the "Featuring" box:

From the sharing screen, open the "Featuring" menu from the right hand side of the page. In the dialogue box, start typing an athlete or multiple athletes names and press "enter." A button displaying each athlete that you tag will appear in the dialogue box. You may select as many relevant athletes as you would like.