@Jamo 

I think the challenge here is that you have a 1 to many relationship between the item in the list and the 1 or more users in the person column that allows multiple selections.

I just did a quick test on a form.

I created a data source against the list in the automate settings. This just has title and staff (multi person) columns.

On the nintex start form I added a data lookup control to this data source.

It is showing the title column for Label and value.

I created an external data variable in the form to the same data source but set the condition to filter based on title from the data lookup control.

I added a label to the form and set it to display the external data / first record / staff (email)

This displays a comma seperated string of emails from the staff field.

It’s text though not a collection variable so will need to dealt with differently from a collection.

The other option is to normalise this into a second sharepoint list, where the second list has only 1 item per staff person. So you would create a 1:many relationship between the first list and the second list.

Hi @Jamo 
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