Log into your dashboard as the administrator and click on the "Invite team" button. The pop-up box will require you to fill out the member's name and email address. Once all details are filled out, click on the "Send invitation" button.
Once the user is set up, you can use the cog icon next to their name to assign them to a plan. You can choose from a Team hours membership or a monthly plan like Explorer, Creator or Luminary.