After experiencing this problem, I inspected the permissions within the Survey123.arcgis.com site's page for the survey, and sure enough, the permissions within the collaboration tab warned that I was the only one able to view/submit. Assigning permissions to the appropriate group there fixed this problem. @James Tedrick: it's kind of weird having permissions in both AGOL and on the Survey123 site, and it seems like they're not always in sync.
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