Open your Team from the Team list on the left.
Select the three dot options menu for the Team you want to add to:
Select Add Member from the options list:
Start typing in the email address or full name of the person you wish to add. Teams will provide a selection based on what you have typed in, just select the correct person and click the add button:
Once that person is added, you can either leave them a member, or make them an owner. Owners have full control of the team, including being able to add/remove members and rename or delete Team sub-teams.