There are few ways to see how many guardians have not registered. You can see this in the Coach HQ and Director HQ

In the Coach HQ: 

You go to the Age group and click on About and there you can see Players without Guardians. If you hover over the information button then you can see their names.

In the Director HQ

1. Click on Age groups and then Stats

2. If you click on Advanced you can see more informations. Then you can see how many players there are without registered guardians.

3. To see their names you click on the number (in this example there are 12 guardians that have not registered).