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.
In case no profile is found the system offers you to invite new team mates to play.fiba3x3.com - this can be done by clicking on the "Invite your teammate to get a FIBA 3x3 profile" link. Once you have entered all necessary information those players will receive an email with further instructions.
If you have either added the information or selected an existing profile you'll need to click on "Send Invitation" and the player/invitation 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.