On the Data and Analytics screen, you can find "User Overview". This gives you an idea of how many students and responders are active within your network. If you have students or responders that are "inactive", this means that they have not officially setup their account.
If this is the case, go to the "Students" and/or "Responders" screen, find the status column, and identify the student or responder who is "inactive".
You can then resend the email invitation to them by selecting "Invite".