In the Update Record action, include the existing value from the record plus the new value.

So structure like this?

The above doesn’t always work unfortunately. When a new record comes in with that field empty then it throws an error because the field is empty and then the automation doesn’t run at all. Am I doing this wrong?

Hum. That is a problem. I recommend contacting support to explain the issue.

In the meantime, you could have two different automations and some formula fields for the the “when record matches conditions” trigger. The formula field would compare the last updated time of the department field with the last updated time of the collaborator field to determine if an automation needs to run.

Automaton A would trigger when the formula field says an updated is needed and the collaborator field is empty. This update would have only the new collaborator.

Automation B would trigger when the formula field says an update is needed and the collaborator field is not empty. This update would have the new collaborator and the list.

Did you find a solution for this? I'm having the same issue

I think I found a solution to this, at least one that works for me for updating fields without overwriting: 

I have a conditional statement that includes "if {field i want to update} is empty" which will simply update the field with the result of the trigger step only and "if {field I want to update} is NOT empty" it will update the field with what is ALREADY in that field, then the result of the trigger, and that seems to take care of the error code 

So something like this: 

(note what's already in the field MUST go first, otherwise if the result of the trigger is empty it throws an error code anyway)