How do I add photos to an existing event? I must be missing something obvious.
Be sure you’re not confusing an “Event” with an “Album”. An Album is an arbitrary set of chosen photos. You can “Add” photos to an Album. An Event is a time/date span - any photos within those dates are part of the Event, unless excluded via an Event Filter selection.
You’d either need to change the dates of your photos to fit within the Event span - or expand the Event span dates to include those photos.
I just adjust the dates. Sometimes if I edit a photo (or apparently my camera clock is often off by 8-12 hours) it needs adjusted to the actual date the event occurred. That will automatically add it. Changing dates is very easy in Mylio.