Eu voltei dessa reunião cedo.

Yes. In Brazilian Portuguese, eu is often optional because the verb form already shows the subject.

• Eu voltei dessa reunião cedo.
• Voltei dessa reunião cedo.

Both mean the same thing: I came back / returned early from that meeting.

Including eu can:

• add emphasis,
• make the sentence clearer,
• sound more natural in some contexts, especially in spoken Brazilian Portuguese.

So eu is correct, but not always necessary.

Voltei is the first-person singular preterite form of voltar.

So:

• voltar = to return / to come back
• voltei = I returned / I came back

This is a completed past action. It refers to something that happened and is finished.

Examples:

• Voltei para casa. = I came back home.
• Voltei ontem. = I returned yesterday.

In your sentence, it means I came back / returned from the meeting.

Because voltar here needs the idea of from:

• voltei dessa reunião = I came back from that meeting

The word dessa is a contraction of:

• de + essa = dessa

So:

• de = from / of
• essa reunião = that meeting
• dessa reunião = from that meeting

This contraction is very common in Portuguese.

Other similar contractions:

• de + este = deste
• de + esta = desta
• de + isso = disso
• de + aquele = daquele

Good question. Both are possible in Portuguese, but they mean slightly different things.

• da reunião = from the meeting
• dessa reunião = from that meeting

So dessa points to a specific meeting, usually one already mentioned or understood from context.

Compare:

• Voltei da reunião cedo. = I came back from the meeting early.
• Voltei dessa reunião cedo. = I came back from that meeting early.

Using dessa adds a demonstrative idea, like that in English.

Here, cedo is an adverb, meaning early.

It modifies the verb voltei, telling us when the person returned.

• Voltei cedo. = I came back early.

It is not describing a noun here, so it is not acting like an adjective.

A very common contrast is:

• cedo = early
• tarde = late

Examples:

• Cheguei cedo. = I arrived early.
• Saí tarde. = I left late.

Because Portuguese often places adverbs like cedo after the main information, especially in simple everyday sentences.

So this word order sounds natural:

• Eu voltei dessa reunião cedo.

You may also hear:

• Eu cedo voltei dessa reunião. — much less natural in everyday speech
• Cedo, eu voltei dessa reunião. — possible, but it gives cedo more emphasis

The most neutral and natural order is the one in your sentence.

They express different directions:

• voltar de = to come back from
• voltar para = to go back / return to

Examples:

• Voltei da reunião. = I came back from the meeting.
• Voltei para casa. = I went back home.

You can even use both in the same sentence:

• Voltei da reunião para casa. = I came back from the meeting to home / I went home from the meeting.

In your sentence, dessa reunião answers from where?

The word reunião is spelled with ão, which is a very common ending in Portuguese.

• reunião = meeting

The mark over the a is a tilde: ã. It shows that the vowel is nasal.

So reunião is pronounced roughly like:

• reh-oo-nee-OWng
  but with a nasal sound at the end, not a fully pronounced English ng

This ending appears in many nouns:

• nação = nation
• opinião = opinion
• situação = situation

It is an important spelling and pronunciation pattern in Portuguese.

Yes, reunião is a feminine noun.

You can tell because it takes feminine articles and demonstratives:

• a reunião
• essa reunião
• dessa reunião

Many Portuguese nouns ending in -ão are masculine, but not all. So with reunião, you simply need to learn it as feminine:

• a reunião

Examples:

• A reunião começou. = The meeting started.
• Essa reunião foi longa. = That meeting was long.

Not literally, but in context it can feel close.

The sentence Eu voltei dessa reunião cedo literally means:

• I came back from that meeting early
• I returned from that meeting early

This suggests that the speaker got back early after being at the meeting.

If you specifically want I left the meeting early, Portuguese would more directly say:

• Eu saí da reunião cedo.

Difference:

• saí da reunião cedo = I left the meeting early
• voltei da reunião cedo = I came back from the meeting early

Depending on context, both may imply that the meeting ended early for the speaker, but the verbs focus on different actions.

Your sentence is already natural. A few very natural variations are:

• Voltei dessa reunião cedo.
• Eu voltei da reunião cedo.
• Voltei da reunião cedo.

Which one sounds best depends on context:

• with eu → slightly more explicit or emphatic
• with dessa → points to that specific meeting
• without eu → very common in conversation
• with da instead of dessa → more neutral, less pointed

So the original sentence is perfectly good Brazilian Portuguese.