I decided to write this up because I have been following a similar post about this issue for a couple of years and people are still asking how to do this. You can certainly go down the road of creating a custom action to convert the list collector to an array but it's really not necessary.
Simply do a look up of the records which will then allow you to loop through them. Ex,
Step 5 - look up user records where sys_id is one of....pass in the list collector variable
Step 6 - if count is greater than 0 we found some users
Step 7 - for each loop - pass in the records returned from step 5
As you can see this is very simple to do with OOB actions. Could probably even turn this into a subflow if needed.
