If staff or students don't appear in Firefly, it's likely down to email addresses not being present or in the correct field.You'll need to ensure that each user has their email address added to their record within your MIS. You'll also need to be sure the address is in the correct field.
If only a few users are missing, it's a good idea to compare the working users to the non-working users to spot the difference easily.
If none of your users are appearing in Firefly then you should follow this documentation on configuring your MIS.
If your school's students don't make use of email addresses for Firefly authentication and they aren't appearing in Firefly, you should contact Firefly support.