الجميع في البيت الليلة.

Because this is a nominal sentence in Arabic. In the present tense, Arabic often leaves out the verb to be.

So:

الجميع في البيت الليلة
literally looks like: everyone in the house tonight

But in natural English, you translate it as: Everyone is at home tonight or Everyone is in the house tonight.

In Arabic, the is/are is understood.

الجميع means everyone, everybody, or sometimes all depending on context.

In this sentence, it means everyone.

It is a very common word for talking about a whole group as one unit.

Because الجميع is a fixed noun meaning everyone / everybody.

Compare:

• جميع = all, entire, often used with another noun
• الجميع = everyone / everybody

For example:

• جميع الطلاب = all the students
• الجميع حاضر = everyone is present

So the الـ here is part of the normal form of the word.

It is grammatically singular, even though its meaning refers to many people.

That means in careful MSA, it usually takes singular masculine agreement.

For example:

• الجميع حاضر = Everyone is present

This can feel strange to an English speaker, because English often thinks of everyone as semantically plural. But in Arabic grammar, الجميع is treated as singular.

Yes. Literally, في البيت means in the house or in the home.

But in many contexts, it corresponds naturally to English at home.

So depending on context:

• في البيت = in the house
• في البيت = at home

Arabic and English do not always use prepositions in the same way, so في here is completely normal.

It can mean either, depending on context.

• البيت literally means the house
• but it very often also means home

So this sentence could mean either:

• Everyone is in the house tonight
• Everyone is at home tonight

If you want a slightly more formal word for home/house, you can also use المنزل:

• الجميع في المنزل الليلة

That is also correct, though البيت is very common and natural.

الليلة means tonight.

Here it is a time expression, and Arabic very often places time expressions after the main statement.

So the structure is basically:

• الجميع = subject
• في البيت = predicate / location
• الليلة = time

That word order is very natural.

You could move the time phrase for emphasis, but the given order is the most neutral:

• الجميع في البيت الليلة

Because الليلة by itself commonly means tonight.

Meanwhile:

• الليلة = tonight
• هذه الليلة = this night / this evening / tonight depending on context

In simple everyday-style MSA, الليلة is a very normal way to say tonight.

In fully vocalized MSA, you could read it as:

الجميعُ في البيتِ الليلةَ

Here is why:

• الجميعُ: nominative, because it is the subject of the nominal sentence
• البيتِ: genitive, because it comes after في
• الليلةَ: accusative, because it functions as an adverb of time

In normal unvoweled writing, these endings are not shown, and in pause they are often not pronounced fully.

A careful MSA reading is approximately:

al-jamīʿu fī l-bayti al-laylata

If you stop at the end, the last word is often heard more like:

al-jamīʿu fī l-bayti al-laylah

A few helpful pronunciation notes:

• جميع has a long ī sound: jamīʿ
• في is fī
• بيت is bayt
• final ة is pronounced like -a / -ah when stopping

Sometimes, but not in exactly the same way.

كلّ usually needs something after it, or a pronoun attached to it. For example:

• كلّ الناس في البيت الليلة = All the people are at home tonight
• كلّهم في البيت الليلة = All of them are at home tonight

But الجميع can stand on its own very naturally:

• الجميع في البيت الليلة

So in this sentence, الجميع is a very convenient choice because it already means everyone by itself.