Learn Hardcore Indonesian: Dalam rapat yang berlangsung pagi ini, peran setiap anggota dijelaskan. - In the meeting that took place this morning, each member’s role was explained.

Indonesian often uses the passive to foreground the thing affected and to sound formal/neutral. Here, the focus is on the content (the roles), not the speaker.

Active: Dalam rapat yang berlangsung pagi ini, ketua menjelaskan peran setiap anggota.
Passive: Dalam rapat yang berlangsung pagi ini, peran setiap anggota dijelaskan (oleh ketua). Both are correct; choose passive when the agent is unknown/irrelevant or when you want a report-like tone.

Who is doing the explaining? How do I say it?

You can add an agent with oleh:

…, peran setiap anggota dijelaskan oleh ketua. You can also use “passive type 2” (no prefix on the verb, agent right after the subject):
…, peran setiap anggota ketua jelaskan. In casual speech, you may hear sama instead of oleh: … dijelaskan sama ketua.

What does yang berlangsung do? Can I omit it?

Yang berlangsung introduces a relative clause modifying rapat: “the meeting that took place.” You can omit it by reshaping the noun phrase:

Dalam rapat pagi ini, peran setiap anggota dijelaskan. (shorter, very natural)
Dalam rapat yang berlangsung pagi ini, … (slightly more explicit/formal)

Is yang necessary? Could I say Dalam rapat berlangsung pagi ini?

You need yang to turn berlangsung pagi ini into a clause that modifies rapat. Without yang, berlangsung becomes the main predicate and the structure breaks. So keep yang or use the simpler rapat pagi ini.

Where can I put pagi ini? Do I need pada?

Time words usually don’t need a preposition.

Natural: Dalam rapat pagi ini, …; Dalam rapat yang berlangsung pagi ini, …
Acceptable but stiffer: … yang berlangsung pada pagi ini Avoid pada pagi ini in everyday style unless you want a very formal tone.

Should I use pagi ini or tadi pagi?

Both can refer to the same morning. In conversation later in the day, tadi pagi often sounds more natural (it emphasizes pastness). In news or reports, pagi ini is common even if published later.

Why dalam rapat and not di rapat, pada rapat, or selama rapat?

dalam rapat = within the proceedings/content of the meeting (very common).
di rapat = at the meeting (colloquial).
pada rapat = at the meeting (more formal/official).
selama rapat = during the meeting (focus on the time span). All can work; choose based on nuance and register.

Do I need to pluralize peran to peran-peran?

No. With setiap (“each”), the noun stays singular: peran setiap anggota. Reduplication (peran-peran) is usually unnecessary (and odd) after setiap. If you drop setiap and talk about roles in general, peran para anggota is fine.

What’s the difference between setiap anggota and masing-masing anggota?

Both mean “each member.” Setiap is neutral and common; masing-masing adds a bit more emphasis on individuality.

Peran setiap anggota dijelaskan.
Peran masing-masing anggota dijelaskan. You can also use the pronoun form: Peran mereka masing-masing dijelaskan.

Can I start the clause with the verb, like … dijelaskan peran setiap anggota?

Yes. Dalam rapat …, dijelaskan peran setiap anggota is grammatical and common in formal reports. It highlights the event/action first. Your original order (Subject + Passive Verb) is equally natural.

Is the comma after the introductory phrase required?

It’s recommended for readability, especially with a long fronted phrase: Dalam rapat yang berlangsung pagi ini, … You’ll also see it omitted in short phrases, but keeping it is safe.

Difference between berlangsung, diadakan, and terjadi here?

berlangsung = to take place (neutral, focuses on the event unfolding).
diadakan = to be held (some organizer implied): rapat yang diadakan pagi ini.
terjadi = to happen (often for incidents/accidents), less natural for planned meetings.

Can I say menjelaskan tentang peran or should it be penjelasan tentang peran?

Use:

menjelaskan peran … (no tentang) or
memberikan/menyampaikan penjelasan tentang peran … Pair menjelaskan directly with its object; use penjelasan tentang when you nominalize.

How do I say “already/clearly/in detail explained”?

Completed: sudah dijelaskan (neutral), telah dijelaskan (formal).
Clearly: dijelaskan dengan jelas (common, though somewhat tautological), better: dijelaskan dengan gamblang.
In detail: dijelaskan secara rinci/terperinci.
Made clearer: diperjelas; active: memperjelas.

Any casual variants of this sentence?

Yes, in everyday speech:

Di rapat pagi ini, ketua jelasin peran masing-masing anggota.
… peran setiap anggota udah dijelasin (sama ketua). Colloquial forms include jelasin (for menjelaskan) and udah (for sudah).

Is rapat the best word, or could I use pertemuan or sidang?

rapat = meeting (often organizational/office context).
pertemuan = meeting/meet-up (broader, can be informal).
sidang = session/hearing (formal, legislative/court/academic). Choose the noun that matches the context of the event.