From your Enterprise account homepage, click on Go to Groups:

Select the group you want to add the kid to and if you're the guardian of the students, simply use the Add Existing Kids option under the Manage Kids menu:

If the students are part of a club, click on the More Tools dropdown and select Add Club Kids:

Students can click on the icon next to their avatar on the homepage to go to Settings

From their Account Settings menu, scroll down and select the Do you have a Group Registration Key? option

Then they can input the provided registration code and click Register

New kids can create a new account directly in the group with a Gold membership by using the group’s Signup link. You can find this link by going to your group and clicking on the Info tab