How to add a new member to a team
Select the team in the upper left corner, go to Team Settings (the little gear icon in the bottom left corner) and select Members. Add an email address or multiple, set their role, and send the invite. Done! :)
It's important to note that all new team members invited have to be approved by your Team Owner or Team Admin. Once one of them approves the request the new member receives an email invite to either create an account (if they don't already have one) or to simply log in and access the team you added them to.
How to approve a member request
As a Team Owner and Team Admin, you will receive all your team's requests for adding new team members and you'll have to approve or reject the requests.
An important thing to note is that each role has a different set of capabilities and that Viewers are always free (they can view and comment), and Editors are paid roles (they can create almost anything).
for more details, you can see the Roles and rights guide.