When on your Team Management page in the right hand side bar you can enter the email address of a user to invite them to join your team:

The user will receive an email with a link to join the team. Once accepted they will be added to your team table on the management page.

Users that have been invited but have yet to click the link in the email will appear as pending users:

You can reissue invitations from the Pending list by clicking on the mail icon, or cancel the invitation by selecting the X button.