First, you need to get to the team registration page via one of the three ways explained in this Help article
When you click Add player, you should receive a dialogue box looking like this
Point your mouse to the box and start typing your teammate's name. Make sure you type the name correctly (Firstname Lastname).
In case no FIBA 3x3 Profile is found, you can access the form under ‘Create a temporary FIBA 3x3 profile on behalf of your teammate.’
Note! You should create a temporary profile on behalf someone with their explicit permission. You will not be able to add any contact information such as email phone number. You will provide just the minimum set of information (First Name, Last Name, Gender Nationality, Hometown, Birthdate) that enables the said player to claim the temporary profile into their FIBA 3x3 profile. Some of the said information above will be obfuscated when displayed publicly.
If you have either added the information or selected an existing profile you'll need to click on "Add Player to Team / Send Invitation" and the player will be added to your team roster.
In any case do note that you can always contact the organizer directly as they will be able to make team roster changes as well. You can find the contact details on the event website in the "About"-Tab.