The Collaborators page allows you to invite people to and remove people from your Organization.

To add a new Team Member to your Organization, use the Menu in the top right corner, and select Add Collaborator To Organization. 

Next, select which Permission level you'd like the person to have—Admin, Editor, or Viewer.

Then, QuickMail will generate a custom link for you to send to your new Team Member. Copy and send it along to them. This is what they'll see when they follow that link:

They'll then be able to join the team, using a Google, Outlook/Exchange Online, Facebook, or LinkedIn account.

Any Pending invites to your Organziation will be displayed at the top of this page. You're able to revoke the invitation, using the button on the right side: