Hi Natasha,
Create yourself a text variable (something like textEmailAddresses).
As you are using a For Each action to go through the collection, run your query and then have a Build String action that does the concatenation for you.
Your Build String action would look something like this in the configuration:
{WorkflowVariable:textEmailAddresses};{WorkflowVariable:textNewEmailAddress}
and store the result back in the textEmailAddresses variable.
What this is doing, is appending a semi-colon and the new email address to the existing variable.