Questions & Answers about Iedereen is welkom op het feest.
Why does iedereen take the singular verb is?
Just like the word 'everyone' in English, the Dutch word iedereen is grammatically singular. That is why it uses is instead of the plural zijn.
Why do we use the preposition op here to mean 'at'?
In Dutch, the preposition op (which literally means 'on') is the standard word to use for events and gatherings. You will often hear phrases like op het feest (at the party) and op een bruiloft (at a wedding).
Could I use allemaal instead of iedereen in this sentence?
Does the adjective welkom ever get an -e added to the end?
Not in this sentence, because it comes after the verb. In Dutch, adjectives usually only get an extra -e when they are placed directly in front of a noun, for example in een welkome verrassing (a welcome surprise).