After initially submitting a case, its information must be updated and an initial case task must be created before adding exposed contacts to the record.
Update a Case
Follow the View Cases Assigned to Me guide to navigate to your assigned cases, and select the case record to update.
Once on the case record, verify that all current field information is correct and sufficient.
Click Start Work at the top right of the screen. This changes the Case State from Assigned to Work in Progress.
Follow the instructions on the Create a Case Task guide to create an initial case task for the reported person.