syusaisya ha mou syuugoubasyo ni tuita you da.

は marks 主催者 as the topic of the sentence.

So the sentence is talking about the organizer, and then saying something about them: that they apparently have already arrived at the meeting place.

A rough breakdown is:

• 主催者は = as for the organizer / the organizer
• rest of sentence = apparently has already arrived at the meeting place

If you changed は to が, the nuance would shift a bit. が would sound more like identifying the organizer specifically as the one who seems to have arrived.

もう here means already.

So it adds the idea that the arrival has happened sooner than expected, or that by now the organizer has arrived.

Examples of the nuance:

• もう着いた = already arrived
• まだ着いていない = has not arrived yet

In this sentence, もう naturally goes with 着いた to mean has already arrived.

集合場所 means meeting place or gathering point.

It is made of:

• 集合 = gathering, assembling
• 場所 = place

So 集合場所に着いた means arrived at the meeting place.

The particle に marks the destination of movement with verbs like 行く, 来る, and 着く.

Since 着く means to arrive, the place where someone arrives is marked with に:

• 駅に着く = arrive at the station
• 家に着く = arrive home
• 集合場所に着く = arrive at the meeting place

So 集合場所に means at/to the meeting place as the destination of arrival.

Yes. 着いた is the past tense of 着く, which means to arrive.

• dictionary form: 着く
• past plain form: 着いた

So:

• 着く = arrive / will arrive
• 着いた = arrived

In this sentence, 着いた tells us that the arrival is seen as already completed.

Because ようだ is describing what seems to be true, and what seems to be true here is that the organizer has already arrived.

So the whole idea being judged is:

• 着いた = has arrived

Then ようだ is added to mean:

• 着いたようだ = it seems that they have arrived

If you said 着くようだ, it would mean something more like it seems they will arrive / are about to arrive, which is a different time reference.

So the tense before ようだ matters:

• 着いたようだ = seems to have arrived
• 着くようだ = seems likely to arrive / seems to be arriving soon

ようだ here means it seems that... or it appears that...

It is used when the speaker is making a judgment based on some evidence or observation.

So:

• 主催者はもう集合場所に着いたようだ。 = It seems the organizer has already arrived.

This usually suggests the speaker is not stating it as a direct confirmed fact, but as an inference based on something they noticed, such as:

• seeing the organizer’s bag there
• hearing from someone nearby
• noticing a message or some sign of their arrival

They are similar, but not exactly the same.

• ようだ: seems / appears; fairly neutral and common in written or spoken language
• みたいだ: seems / looks like; more casual and conversational
• らしい: seems / I hear that / apparently; often based more on hearsay or typical characteristics

In this sentence:

• 着いたようだ = it seems they arrived
• 着いたみたいだ = looks like they arrived (more casual)
• 着いたらしい = apparently they arrived / I hear they arrived

So ようだ is a good neutral choice.

Yes. だ is the plain copula here, attached to よう to make ようだ.

This gives a plain-style sentence:

• 着いたようだ。

In more polite speech, you would usually say:

• 着いたようです。

So:

• ようだ = plain
• ようです = polite

The meaning is basically the same, but the level of formality changes.

It is somewhat neutral/plain.

Why:

• 着いた is plain form
• ようだ is also plain form
• the vocabulary itself is not especially casual slang

So this kind of sentence could appear in:

• narration
• written reports
• neutral spoken explanation
• internal thoughts

If speaking politely to someone, you would more naturally say:

• 主催者はもう集合場所に着いたようです。

The structure is:

• 主催者は = as for the organizer
• もう = already
• 集合場所に = at/to the meeting place
• 着いた = arrived
• ようだ = it seems

So the pattern is:

[Topic] は [time/adverb] [destination] に [verb in past/plain form] ようだ

A natural English gloss is:

The organizer seems to have already arrived at the meeting place.

Not word-for-word, but that is the natural sense of the Japanese sentence.