Organisations can very rapidly collate lists of those who have been involved, often from Incident Log Book and staffing rotas.
The stresses and worries for First Responders are often that of feeling overwhelmed or overloaded in the first instance. They have arrived there with a small number of resources, but everything still needs to be done.
This can lead to either:
freezing - being overwhelmed and doing nothing
trying to do everything at once, because you know it all needs done
triage of working load and prioritising of tasks (this reaction is often from those who are trained and experienced in incident management)
Don't forget there will be staff who are not directly involved in the incident 'those who are still operating in the business as usual domain' who may still be affected.
Remember the aim of this is to do the "most for the most"