Learn Hardcore Indonesian: Kami bergabung dengan komunitas belajar bahasa Indonesia di kota. - We join an Indonesian-learning community in the city.

Kami = “we” (excluding the listener). Use it if the person you’re speaking to is not part of the group.
Kita = “we” (including the listener). Use it if you’re talking to someone who is also part of the group. In your sentence, kami implies the listener is not in that community. If they are, say kita.

Why is it bergabung dengan, not bergabung ke or just bergabung + noun?

The standard, most natural collocation is bergabung dengan + [group].
Colloquially, you’ll hear gabung ke/sama + [group] (dropping the prefix and using ke/sama).
Bergabung ke is heard but feels less standard/formal than bergabung dengan.
Avoid bare bergabung [noun] in formal Indonesian.

Can I use gabung instead of bergabung?

Yes, gabung is common in casual speech:

Formal: Kami bergabung dengan komunitas…
Casual: Kami gabung ke/sama komunitas… Use bergabung in writing or formal contexts; gabung in everyday conversation.

Could I say ikut, masuk, mendaftar, or menjadi anggota instead? Any nuance differences?

Ikut = take part/join (broad, very common): Kami ikut komunitas…
Masuk = enter/join (focus on entry): Kami masuk komunitas… (casual)
Mendaftar = register/sign up (the act of registering): Kami mendaftar ke komunitas…
Menjadi anggota = become a member (status change): Kami menjadi anggota komunitas… Use the one that matches the aspect you want to emphasize.

What exactly does komunitas belajar bahasa Indonesia mean structurally?

It’s a noun phrase where a verb phrase modifies the noun:

komunitas [yang] belajar bahasa Indonesia = “a community [that] learns/studies Indonesian.” Indonesian often drops yang in such “reduced relative” structures. You could also say:
komunitas pembelajar bahasa Indonesia = “a community of Indonesian learners” (focus on the people as learners).

How should I capitalize bahasa Indonesia?

In Indonesian orthography (PUEBI):

Write bahasa Indonesia (lowercase b in bahasa, uppercase I in Indonesia). In English texts, you may see Bahasa Indonesia, but when writing Indonesian, prefer bahasa Indonesia.

How do I show past, present, or future time with this sentence?

Indonesian uses particles/adverbs:

Past/Completed: Kami sudah/telah bergabung…
In progress: Kami sedang bergabung…
Future: Kami akan/ingin/berencana bergabung…
Just did: Kami baru saja bergabung… Base sentence without markers is time-neutral and relies on context.

What does di kota modify here—the community or the act of joining?

By default it’s read as the community’s location: “the community (located) in the city.”
To make it explicit:

Should I make di kota more specific (this city, that city, our city)?

Use:

di kota ini (this city), di kota itu/tersebut (that/said city), di kota kami (our city), di kota setempat (the local city/town), di pusat kota (in the city center).

Any difference between komunitas, kelompok, and grup?

Komunitas: a community around a shared interest; sounds organized; neutral–formal.
Kelompok: a group/set/team (often task-based or categorization).
Grup: “group,” often for chat/social media groups; casual. Choose based on setting: komunitas for an interest-based organization; grup for a WhatsApp/Facebook group.

How do I express “a community,” “the community,” or “several communities”?

“A community”: sebuah komunitas or just komunitas (article often omitted).
“The community”: komunitas itu/tersebut.
“Several communities”: beberapa komunitas, banyak komunitas, or komunitas‑komunitas (reduplication).

Is di kota the only correct preposition? What about ke, dari, pada, dalam?

di kota = in the city (location) ✓
ke kota = to the city (movement) ✓
dari kota = from the city ✓
pada kota = unnatural for physical location ✗
dalam kota = within the city (as a domain), e.g., transportasi dalam kota; for location, say di dalam kota.

Any quick pronunciation tips for this sentence?

e in kami, dengan, bergabung is a schwa in de‑ngan, ber‑ga‑bung: roughly /dəŋan/, /bərgabʊŋ/.
g is always hard, like “go” (never like “gem”).
ng = single sound /ŋ/ (as in “sing”).
r is a tap/flap.
u = “oo” in “food”; a = “a” in “father.”
Natural syllables: Ka‑mi ber‑ga‑bung de‑ngan ko‑mu‑ni‑tas be‑la‑jar ba‑ha‑sa In‑do‑ne‑sia di ko‑ta.

Is there a casual alternative for everyday speech?

Yes, for example:

Kita gabung ke grup belajar bahasa Indonesia di kota. (if including the listener)
Kami ikut komunitas belajar bahasa Indonesia di kota. (neutral, common)