As part of the migration, all users, escalation contacts, managers, and admins are migrated to Nexus on your behalf.
If people are not appearing in Nexus, their profile on the legacy Peoplesafe portal did not have sufficient information to migrate to Nexus.
What are the minimum requirements for profiles by role?
You can collect the information from the person and add them through the people page, read these articles for help.
How to add and activate a User
How to add an Escalation Contact