If you're a team owner, go to the team name in the teams list and click the three dots More options > Add member.
Start by typing a name, or email address. If permissions allow, you can add people outside of the organization as guests by typing their email addresses.
When you're done adding members, select Add.
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.
Microsoft Teams can have multiple owners. Team owners have the ability to add or remove members and create discussion Channels. Below is how to promote a Team member to a Team owner.