As an admin, you can invite anyone to your team via the Members tab of your Admin Dashboard (for Enterprise admins) and the Members tab of your Team page (for Pro-Team admins).
Click on 'Members' and then on 'Add Member'
Type in the email address of the person you want to add to your team.
Click on 'Invite'
The new team member will receive an email and invitation link. Once they accept, they will be part of your team.