We hold design events early in the design process where non-designers and designers real-time co-edit a file. This takes up at most 4 days a year for a non-designer. After that step, designers pick up the design file to continue iterating, and the non-designers don't usually touch it again.

Right now, we use Balsamiq Cloud, which has an enterprise license model where there's a limit on the number of active projects, but the number of collaborators is unlimited. This works really well for these design events (not to mention being browser-based means an appropriately small setup time for 4-times-a-year use).

I saw that Starter XD allows co-editing a file with one other person and that the Single App license allows co-editing a file with unlimited people. It's unclear to me if someone with Single App shares their file with multiple people with Starter, if everyone can co-edit that file now.

It's fine if the Starters can only participate in one file at a time and need to be kicked/added to a file for a design event. It's unclear if this setup is enabled by the current licensing model.