I clicked "aku bersama-sama dengan mereka" in the multiple choice and it counted it is correct, but when I click on the discussion tab it shows a different answer.
Duo is using a database that allows you to use a different answer which would be also correct. The downside is the procedure uses all the options to create choice answers, so you might meet an answer you didn't learn yet.
Why is it bersama-sama?
Because it's plural. Bersama can be used only in singular, like "kamu" or "dia". If you want to use it for plural, it is necessary to use plural form "bersama-sama" or "berkumpul".
what is the sama
As an adverb, "sama" means "together" or "jointly".
It has variety of meanings in a different part of speech.
What is the difference between berkumpul and bersama?
What wrong with "Saya bersama dengan mereka"?
Bersama can't be used for plural, plural would be "bersama-sama" or "berkumpul"
Berkumpul means gathering
Bersama means together
If Iam not wrong they should Chang it