Send an invitation
Go to Settings.
Under Add Members, enter their Email.
Pick a Role (see below).
Select Add. The form shows Added.
The invitee receives an email with an invitation link.
The roles:
Member: Can do everything within your organization.
Team Admin: Full access, including organization settings and billing.
Accepting the invitation
The invitee opens the link while logged in with the email address you invited, or signs up with that address first. Opened from another account, the link only opens that account's settings and the invitation stays open.
Pending invitations
Until they accept, the invitee is listed under Pending Invitations. Select Cancel next to an invitation to withdraw it.