Learn Hardcore Indonesian: Sebagian peserta sudah pulang, tetapi para panitia masih sibuk. - Some participants have already gone home, but the committee members are still busy.

Sebagian peserta = some of the participants (a part of the whole group).
Beberapa peserta = a few/several participants (focus on a small number, not on the idea of a portion). Related:
sebagian besar = most
sebagian kecil = a small portion

Do I need dari after sebagian? As in sebagian (dari) peserta?

Both are acceptable:

Sebagian peserta and sebagian dari peserta are both natural. Adding dari makes the partitive feel a bit more explicit.
With pronouns, sebagian dari is more common and natural: sebagian dari kami/kita/mereka. You may also see sebagian kami, especially in formal writing, but sebagian dari kami is very frequent in speech.

Why is there no plural ending on peserta?

Indonesian nouns are number-neutral. Plurality is shown by context or optional markers:

Numeral: tiga peserta
Human plural marker: para peserta
Reduplication: peserta-peserta (grammatical but uncommon; often avoided in modern style) Here, sebagian already implies a plural subset.

What does para add in para panitia? Can I just say panitia?

Para is a formal/respectful plural marker for humans. Para panitia draws attention to the individual committee members. You can simply say panitia, which (as a collective noun) can mean the committee or the committee members as a group. Alternatives:

anggota panitia = committee members (explicit)
panitia = the committee (collective)
para panitia = the committee members (plural, slightly formal) Note: para is only for people, not things.

Is panitia singular or plural?

It’s a collective noun meaning committee, so number depends on context:

panitia can refer to the committee as a unit or its members collectively.
If you want to emphasize individuals, use anggota panitia or para panitia.

Does sudah mark past tense?

Indonesian has no tense. Sudah marks completion/already:

sudah pulang = have gone home/already went home. Formal alternative: telah. Colloquial: udah. The natural negative counterpart is belum (not yet), not tidak.

What is the nuance of pulang versus pergi and kembali/balik?

pulang: to go home, return to one’s base/origin. Intransitive. sudah pulang = have gone home.
pergi: to go/leave (direction not specified).
kembali (formal) / balik (informal): to return to a previous place (not necessarily home). To send someone home: memulangkan.

Why use tetapi? Could I use tapi, namun, or akan tetapi?

All express contrast, with different registers:

tapi: informal.
tetapi: neutral to formal; good in most writing.
namun: often sentence-initial in formal writing.
akan tetapi: very formal/emphatic. The sentence uses tetapi for a neutral-formal tone.

Is the comma before tetapi required?

It’s standard when joining two independent clauses:

… sudah pulang, tetapi … Informally the comma is sometimes dropped, but keeping it is recommended.

What does masih add in masih sibuk? How is it different from sedang?

Masih means still, indicating continuation from earlier. Sedang marks a current ongoing state/action without implying continuation.

masih sibuk = still busy (and were busy before).
sedang sibuk = currently busy (no claim about before). You can say masih bekerja, masih beres-beres. Masih sedang… is rarely necessary.

How do I negate ideas with sudah and masih correctly?

Negative of sudah is belum: Sebagian peserta belum pulang = some participants have not yet gone home.
To say no longer, use sudah tidak or tidak lagi: Panitia sudah tidak sibuk / Panitia tidak sibuk lagi.
Plain tidak negates without time-aspect nuance: Panitia tidak sibuk = the committee is not busy.

Can I reorder the clauses?

Yes. For example:

Para panitia masih sibuk, tetapi sebagian peserta sudah pulang. To highlight contrasting simultaneous situations, sedangkan works well:
Sebagian peserta sudah pulang, sedangkan para panitia masih sibuk.

What would a casual version sound like?

A natural informal variant:

Sebagian peserta udah pulang, tapi panitia masih sibuk. With pronouns, colloquial Indonesian often adds pada to signal plural:
Mereka udah pada pulang, tapi panitia masih sibuk.

Could I say sebagian besar peserta instead? What’s the difference?

Yes. Sebagian besar peserta means most of the participants (a majority). Sebagian peserta is just some portion, with no majority implied. Also common:

kebanyakan peserta = most participants (colloquial)
sebagian kecil peserta = a small minority

Is para used with any noun? Is peserta-peserta okay for plural?

Para is only for humans or personified beings: para guru, para peserta, para panitia. Don’t use it with inanimates.
peserta-peserta is grammatical but stylistically marked; in modern usage, prefer para peserta or just peserta unless you need a special nuance.

How can I emphasize that it’s the committee members who are still busy?

Use a focus structure with yang or topicalize with -nya:

Yang masih sibuk itu para panitia.
Panitianya masih sibuk. = The committee (the one we’ve been talking about) is still busy.

Does peserta mean participants or attendees? What if they’re just audience?

Peserta are active participants. For audience-only:

hadirin or para hadirin = audience/attendees (formal address)
penonton = spectators (shows, sports)
pengunjung = visitors So for non-participating attendees: Sebagian hadirin sudah pulang.