You will only be able to invite new users to your team if your user role is either an Admin or Manager, review our What do the different roles mean? guide for more information on the different roles.
If you have the correct permissions, Click the "Team" link in the top menu. This takes you to the "Team Management" section.
A left menu should now have appeared, click the "Invite Members" link.
The top half of this page should be an "Invite A Member" box. Simply enter the users email address here and click "Send Invite".
The user will be sent an invite containing a link to accept the invite and join your team.
First navigate to the "Invite Members" page as described above for inviting a new user.
Any pending invitations will be shown below the "Invite a Member" form, showing the email address and the date the invite was sent.
Find the email address that was incorrectly invited, to the right hand side of the email there will be a "Cancel Invite" button. Click this button and then click "OK" to confirm the cancellation.
If nothing is shown below the "Invite a Member" form, either the invite has already been cancelled or the user has already accepted the invite and you will have to remove the user from your team.