As an admin, you can invite anyone to your team via the Members tab of your Team page.

How do I add members to my team?
Click on the initial icon in the top right corner
Click on Team
Go to Members and then on Add Member on the right
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.
How do I add remove members from my team?
On the same Team page as detailed above:
Check the box to the left of the user you want to remove
Click on the delete icon in the toolbar at the top of the page