To add a member to a team to a Team in Teams a user needs to be an owner to add a person to a team.
To check who's an owner or a member:
1. To the right of the team name, click on the ellipsis (the three dots), and select "Manage team."
2. To the right of each user is their role either owner or member.
3. If you’re a team owner, go to the team name in the teams list and click More options > Add member.
4. Start by typing a name, distribution list, security group, or a Microsoft 365 group.
You can also add people outside your org as guests by typing their email addresses.
When you’re done adding members, select Add. You can make someone a team owner by selecting the down arrow next to Member. A team can have multiple owners.
Select Close.
People that you add to a team will receive an email letting them know they are now a member and the team will appear in their teams list.