I found the join(separator, question) function listed on this help page
https://doc.arcgis.com/en/survey123/desktop/create-surveys/xlsformformulas.htm
But when I use it I only get the value in the last repeat. No comma or other repeat data at all. It changes to the last value in the repeat whenever a repeat is added.
I saw some posts of it not being ready yet but that was back from Nov 2017.
Thanks