Dix-huit personnes sont au vide-grenier.

Sont is the they are form of the verb être (to be). Since personnes is plural, the verb must also be plural:

• une personne est = one person is
• dix-huit personnes sont = eighteen people are

So est would be wrong here because it is singular.

Au is a contraction of à + le.

• à can mean at, to, or in, depending on context
• le is the masculine singular definite article (the)

So:

• à le → au

In this sentence, au vide-grenier means at the garage sale / flea market / jumble sale, depending on the translation you were given.

Because vide-grenier is being treated as a noun with the article le: le vide-grenier. When à comes before le, French contracts them:

• à + le = au

So à vide-grenier would be incorrect here.

Literally, vide-grenier means something like attic-emptying:

• vider = to empty
• grenier = attic

A vide-grenier is an event where people sell old things they no longer want, often like a neighborhood yard sale, garage sale, or flea market. The exact English translation can vary by region and context.

It is singular here: le vide-grenier. The sentence means the people are at one such event.

If you were talking about several of them, you would use the plural form, for example des vide-greniers.

French numbers usually go directly before the noun without an article:

• trois chats = three cats
• dix-huit personnes = eighteen people

So you do not say des dix-huit personnes here unless you mean something more specific, like the eighteen people in a special context.

Yes. Personne is a feminine noun in French, even when it refers to male people, female people, or a mixed group. That is why you would say:

• une personne
• deux personnes

But in this sentence, you do not see feminine agreement on an adjective, so it is less obvious.

A simple pronunciation guide is:

deez-weet per-son suhN oh veeed-gruh-nyay

A few useful notes:

• dix-huit is pronounced with a z sound linking into huit
• personnes ends with a soft sound; the final -s is silent
• sont has a nasal vowel, and the final -t is silent
• au sounds like oh
• in vide-grenier, the final -r of vider is not pronounced separately here because it is part of the fixed noun

In careful modern French, you normally do not make a liaison between personnes and sont here. So it is pronounced roughly as:

personnes sont, not personne-z-sont

Some liaisons are required, some are optional, and some are normally avoided. This is one that is generally avoided in everyday French.

Yes. That is very natural French and means There are eighteen people at the garage sale.

The difference is mostly one of structure:

• Dix-huit personnes sont au vide-grenier = Eighteen people are at the garage sale
• Il y a dix-huit personnes au vide-grenier = There are eighteen people at the garage sale

The second version is often more common in everyday speech when you are simply stating that some number of people are present.