The Invitation Workbasket lists all invitations that have been sent out to applicants that have yet to be completed. Once the applicant completes Part I of the enrollment form, the applicant will appear in the Pending Enrollments Workbasket. You also have the ability to complete the enrollment form on behalf of the member through the Invitation Workbasket.
See also:
When should I delete an invitation?
What is the difference between the Invitation Workbasket and the Pending Enrollment Workbasket?