To add members to a Team on an existing entry, first navigate to the registration. An easy way to do this is to click on your name in the top right corner of the window, and then clicking on the dashboard button. On your dashboard click the "View registration" button next to the entry you choose. (You can also access your Dashboard from here https://enter.entryninja.com/dashboard)
Once on your registration, scroll to the bottom of the member section and click the "add team" button.
Enter an appropriate team name and select the members you want to add to the team, then click the "Add" button to create the Team
(Ctrl + Click to select multiple members).