Learn German: Jeder muss die Gebühr bezahlen, außer dem Baby. - Everyone has to pay the fee, except the baby.

The word außer is a preposition that always requires the dative case. Since Baby is a neuter noun (das Baby), the article changes to dem in the dative.

Why does Jeder take the singular verb muss when it refers to a whole group of people?

Just like the word "everyone" in English, the German pronoun jeder is grammatically singular. Even though it means all people, it takes a singular verb. If you wanted to use a plural verb, you would use Alle (All), as in Alle müssen die Gebühr bezahlen.

Why is there a comma before außer?

Normally, you do not need a comma before außer when it simply connects nouns in the middle of a sentence (like Jeder außer dem Baby muss...). However, when the außer phrase is tacked onto the very end of an already complete sentence as an afterthought or clarification, a comma is often used to separate it.

Does außer always mean "except"?

It usually means "except" (excluding someone), like in this sentence. However, it can also mean "besides" or "in addition to" (including someone). You have to tell from the context. For example, Außer dem Baby muss auch der Hund draußen bleiben would mean "Besides the baby, the dog also has to stay outside."