You'll first want to find their registration under "Search and Manage" under "Registrations.
Once you've found who you're looking for open their registration details.
On the right side of the screen under Registration Details, you'll find the Team Name header. If the participant is not on a team the Team Name will display as N/A. If the participant is on a team the Team Name will be displayed and be click-able to the Team Details page.
To add the participant to a team, or to change the team they are currently registered on, simply click the Change button which will open the Team Change overlay window.
In the Team Change window you'll see Select a new Team. Open the select box and start typing in the new team's name you wish to add the participant on to. Once the team is selected simply confirm the section by clicking the blue Change Team button.
Now that you've added the participant onto a new team, you'll notice the Team Category may have changed depending on the team configuration for that event.
IMPORTANT REGISTRATION CATEGORY NOTE: Some events have team-specific registration categories. Once you change the team, especially if you're adding a participant onto a team who was previously not on a team, be sure to review the Registration Category as you may need to change it so there is no conflict. The system will not automatically change the registration category.