في الحفلة كان فيه ناس كتير من العيلة.

Here it means at the party.

The preposition في literally means in, but with events in Egyptian Arabic, it often corresponds to natural English at:

• في المدرسة = at school / in the school
• في الشغل = at work
• في الحفلة = at the party

In speech, في الحفلة is often pronounced something like fil-ḥafla.

كان فيه is a very common Egyptian Arabic way to say there was / there were.

• فيه = there is / there are
• كان فيه = there was / there were

So the structure is not just a normal verb plus subject. It is an existential expression, like English there is / there are.

Examples:

• فيه مشكلة = There is a problem.
• كان فيه مشكلة = There was a problem.
• كان فيه ناس = There were people.

They are doing two different jobs.

• في at the beginning is a preposition meaning in / at
• فيه after كان is the expression meaning there is / there are

So in this sentence:

• في الحفلة = at the party
• كان فيه = there was / there were

Even though they look similar, they are not the same thing grammatically.

Because كان فيه works as an impersonal existential pattern in Egyptian Arabic.

The important idea is that كان فيه as a chunk means there was / there were, and it does not usually change to match the following noun the way an English learner might expect.

So:

• كان فيه ناس = There were people.
• كان فيه مشاكل = There were problems.

This is very normal in everyday Egyptian Arabic.

ناس means people. It is plural in meaning.

It is a very common everyday word and is often used where English uses people:

• ناس كويسين = good people
• ناس كتير = many people

So in this sentence, ناس كتير means a lot of people / many people.

In this kind of phrase, كتير is the normal everyday way to say many / a lot of after a noun.

So:

• ناس كتير = many people
• فلوس كتير = a lot of money
• حاجات كتير = many things

You may also hear other forms in other contexts, but ناس كتير is the most natural everyday phrasing here.

Literally, it means from the family.

In natural English, that usually means:

• from the family
• of the family
• relatives
• family members

So ناس كتير من العيلة means that many of the people there were family members or relatives.

Because العيلة is the everyday colloquial Egyptian word for family.

Compare:

• العيلة = family, in everyday Egyptian speech
• الأسرة = family, but more formal / Standard Arabic
• العائلة = also family, understood, but more formal than العيلة

If you are learning Egyptian Arabic for conversation, العيلة is the most useful word to know.

It can often include extended family too.

In everyday Egyptian Arabic, العيلة is frequently broader than just parents and children. Depending on context, it can include:

• parents
• siblings
• uncles and aunts
• cousins
• relatives in general

So in this sentence, it likely means a lot of relatives / family members were at the party.

Because Arabic often puts the setting first.

Starting with في الحفلة gives the listener the scene first:

• At the party...

Then the sentence tells you what existed there:

• ...there were many people from the family.

This word order is very natural in Arabic.

You could also say:

• كان فيه ناس كتير من العيلة في الحفلة

That is also understandable, but the original version feels very natural because it sets the scene first.

A natural pronunciation would be something close to:

fil-ḥafla kān fīh

A few helpful notes:

• في الحفلة often sounds like fil-ḥafla
• ح in حفلة is a stronger h sound than English h
• فيه is often pronounced fīh

So the full sentence might sound roughly like:

fil-ḥafla kān fīh nās kitīr min il-ʿēla

It is definitely colloquial, and it sounds very natural in Egyptian Arabic, but most Arabic speakers would probably understand it.

A few parts are especially colloquial:

• كان فيه for there was / there were
• العيلة for family
• كتير in everyday speech

So it is Egyptian-style spoken Arabic, not formal Standard Arabic, but it is still quite widely understandable.

A more formal version could be something like:

كان هناك كثير من أفراد العائلة في الحفلة

or

في الحفلة كان هناك كثير من الناس من العائلة

In Egyptian Arabic, though, the original sentence is much more natural for normal conversation:

في الحفلة كان فيه ناس كتير من العيلة