Adding members to a team:

1. Go to the team name and click More options (...).
2. Select Add member.
3. Type the name or email address of the person.
4. If you're a team owner: Choose the person's permissions (Member or Owner), then click Add.
5. If you're not a team owner: Enter the name, and your request will be sent to the team owner for approval.

Managing existing members (for Team owners):

1. Go to the team name and click More options (...)
2. Select Manage team
3. Go to the Members tab
4. Here you can change roles or remove members

For more details, see Microsoft's guide: Add members to a team in Microsoft Teams