You can let in a colleague or an agency too, with the right permissions. (3 steps, about 2 min)
In the left menu, choose Users.

Invite a teammate: enter the email address and choose a role: Editor, Approver or Viewer. At the bottom of the page you can see what each role can do.

Click the Invite button. Your colleague can sign in through the link in the invite. Until then you see them as Invited, and you can withdraw the invite with the Remove button.
