Team Owners can add someone to their team
To add members to a team:
If you're a team owner, go to the team name in the teams list and click More options > Add member.
Start typing a name, distribution list, security group, or Office 365 group to add to your team. You can also add people outside your organization as guests by typing their email addresses.
When you're done adding members, select Add. Fyi, you can make someone a team owner by selecting the down arrow next to the word Member. There can be multiple owners in a team. For a deeper look at roles and permissions, see Team owner and member capabilities in Teams.
Select Close. People you add to a team will receive an email letting them know they are now a member of your team and the team will show up in their teams list.