"Unterlagen" in the sense of "documents"? Yes. According to Duden, it's mostly used in the plural. So it could technically be used in the singular as well, but I've never actually heard the singular. For all intents and purposes, it is a plurale tantum.
"Unterlage" meaning "mat" or "pad" is a regular noun. It can occur in the singular and plural.
This also points out one of those subtle differences in they way British and Americans speak. Here in the US, we tend to use conjugate with collective nouns as singular, whereas in the UK, one is more likely to do so as plural. "The band is angry with their manager" vs. "The band are angry with their manager".
If the contextual translation of this answer allows for "group" to be "class", why would "Die Gruppe lernt." not allow the same? I missed it just a moment ago by trying to use "class".
It might be clearer if Duo just removed the contextual elements from the translation hints and stayed focused on the translation to "group", with perhaps a footnote explanation of the contextual translation possibilities back out on the lesson selection screen.
I put "the group read" and it was marked incorrect. The correct answer is "the group reads." I see that group follows the same format as for he/she/it, i.e. is singular. You would say "he reads" as opposed to "he read" (singular) You would say "the children read," not "the children reads" (plural) So I see my mistake and understand the rule.. but this doesn't translate, in English "the group read" is perfectly acceptable, I am a little confused on the translation and wondering if I understand it correctly!
While the German sentence uses the singular verb form (liest, as discussed in previous comments), English can use the plural verb form: "The group are reading" is the present-tense continuing form, but "The group read" is the equivalent in simple present tense. Think of it as how Sie lesen could be either "They are reading" or "They read".
So, yes, that should be accepted and there's no problem that it is. It is not a typo.
No, we don't use them in English, unless they are a plural form. No one in the United States, England, Australia or Canada says "The group are going out." of "The band are playing here." or "The company are afraid of high prices." They WILL say "The media are circling the wagon." or ""The data do not confirm this." because those are Latin plural nouns. If you want to make a collective noun plural and not get strange looks, then you'll need to qualify the individuals within the collective ("members of the band, people inside the company" etc.....). This inserts a plural noun and makes the collective noun the object of a preposition. Otherwise using a plural verb for a singular collective noun is extremely rare.