All you need to do is select the ‘Invite members’-button on the top header of your screen.
Once clicked, the following screen will pop up:
Insert the email address of the member(s) you want to invite, and click 'Add'.
After you added the member, you can select the role you want this new member to have. There are three options: admin, member and guest.
As soon as you selected the role you want your new team member to fulfill, click 'Invite' and you're good to go!