Select "Track Performance" from your left navigation menu. Use the "Add Data" button of the test you want to run:

Use the "Add Data" button found below the open test:

Now that you're on the test page (in this image, "Body Composition (Skinfolds)" ), select "Add Athletes." You will see a drop down menu of athletes to choose from. If you want to add a whole team, select "Filter by team," and select the team you want. Then select "Add all visible" to select everyone in the list. Selected athletes will be highlighted in blue. If you want to deselect an athlete, click on his or her name again, and the blue should disappear, letting you know the athlete was deselected. 

Once you're finished adding athletes, click anywhere on the page and the athletes menu will disappear. You can add or remove athletes at any time before saving by selecting "Add Athletes" again. Enter the test results in the appropriate box as if this was any other spreadsheet.  And if you already have a spreadsheet with the testing results in a csv format you can use the "Choose File" button to upload your results too!

When ready, "Save" as a draft or "Publish" if complete and no further edits or data are needed.  Once you "Publish" you cannot edit or change the data.