"If you want to help people, then you should join our organization."
Translation:Wenn du Menschen helfen willst, dann solltest du bei unserer Organisation mitmachen.
So Duo doesn't teach you the words for "join." It seems to want you to use "bei... mitmachen" but I'm also trying the words "verinigen" and "verbinden," and it's suggesting "beitreten."
"vereinigen" = "to unite", "verbinden" = "to connect"
"beitreten" works, and in this context I think "mitmachen" doesn't sound very elegant, because you can join an organisation without actually doing something in that organisation. The word "mitmachen" doesn't really imply that you actually do something, but it does have that connotation, because that's what the literal meaning is. "Beitreten" is just declaring yourself as a member (by signing a note, for example).
If you already are a member, you could say, "Ich mache bei der Organisation mit" (but then I'll assume you actually do things there), or "Ich bin der Organisation beigetreten" (perfect tense - "I have joined", so it's really about the initial act of joining). This is just to illustrate how "mitmachen" and "beitreten" work; besides that, I'd preferably just stick with "Ich bin [ein] Mitglied der Organisation" (also: "bei [an organisation without a definite article, e.g. Greenpeace, not e.g. a club or political party]", "bei der / beim / bei den (pl.)", "in der / im")
The English version says "join" which means "beitreten" and NOT "mitmacchen"( to t,ake part in). Also, "if" can be translated as "wenn" oder "falls', but Duo does not accept the latter (yet?)
"falls" (which, unlike "wenn", tends towards "supposing [this happens], ...") sounds a bit doubtful and meek in this sentence, but it's correct, of course.
"ob" is the kind of "if" that's close to "whether": "Sie fragt ihn, ob er Pizza will." = "She asks him if he wants pizza."; "Ich frage mich, ob es regnen wird." = "I wonder if it's going to rain."
The kind of "if" in "If (= in case) you want to help people, then..." translates as "wenn" (or "falls", as Puett pointed out).
The given translation is inconsistent; you could leave out the "then" (German: "dann") in both English and German, or put it in, but I'd much prefer not putting it into one and leaving it out of the other.
Wenn du Leute helfen möchte, dann solltest du unsere Organization beitreten" was not accepted :(, what is wrong??
"Wenn du Menschen helfen willst, dann solltest du unsere Organisation beitreten. " was not accepted, what is wrong?
I found the error: accepted was the following translation "Wenn du Menschen helfen willst, dann solltest du unsereR Organization beitreten."
I also just realised that "beitreten" is a dative verb. Just when you think German is starting to make sense... :P
I thought it might also be worth mentioning that the "s" in "Organisation" was right the first time, and shouldn't have been changed (I presume by typo) to a "z" :)