Be aware though that using "alle" in German may be used in plural rather than singular.
"Alle sind da." translates to "Everybody is there." even though "sind" translates to "are".
In the example I think of it more like "All persons are there." or "All people are there".
It's functionally the same as the -en ending for verbs. I think dropping the e here is just an exception for this word because of the way it's spelled or pronounced. I bet there are at least a few other words that work the same way too. I'm not sure if there's a rule for it, though.