Inviting someone to a roadmap does not automatically add them to your team plan. Editors can invite others to view or edit their roadmaps, however only the team administrator can invite a new person to join a team plan.
To invite someone to just a roadmap, click ‘Share’ on the menu to the right. From there, you can invite someone by email address to view or edit that roadmap. Editors require a paid license, but viewer accounts are free.