That's a bit restrictive. You can use "alguno de ellos" to mean "some of them" "any of them" or "one of them." Perhaps, SpanishDict (I couldn't find the reference) was illustrating what they consider better usage to minimize ambiguity, since "some of them" isn't the same as "one of them."
This is one dictionary entry, specific to the singular form, from RAE:
Expresa un número escaso e inespecífico de personas o cosas. U. referido a un sintagma nominal mencionado o sobrentendido, o para aludir a un sintagma pospuesto introducido por la preposición de.
Expresses a limited or non-specific number of people, animals or things. Used to refer to a noun phrase mentioned or understood, or to refer to a subordinate phrase introduced by the preposition de
I think that is is because both "alguno" and "ellos" don't have to be people. Within this context, this sentence could be talking about places, in which case you wouldn't use the personal "a". Visitar also isn't one of the verbs that always has to halve "a" afterwards, like "ir".
It could also be that they're people, but since the identity isn't important, this doesn't get a personal "a." I'm told that in sentences like "call a plumber" sometimes that plumber basically doesn't count as a person, because you don't care at all about his particular identity beyond "plumber."
Not a specific rule but very generally (depending on context), uno most often refers to an object and alguno refers (most often) to a person. Not to confuse the issue but they can be synonymous but they are not synonyms. It might help to think of "alguno de ellos" as a phrase meaning "one of them (people)".
Not really, that is the actor can often be deduced from the context but you do not like that, with the voice and punctuation (Duo seldom uses ¡!) to separate statements from commands.
"Punctuation to the Rescue!
Because the informal tú command (imperative) is the same conjugation form as the 3rd person singular, it is helpful to include exclamation points to indicate urgency or the name of the person you are commanding followed by a comma. Without the comma, your sentence is just the 3rd person singular present indicative. Compare the following sentences written with different punctuation:
• Compra la camisa. (She buys the shirt.) - present indicative
• ¡Compra la camisa! (Buy the shirt!) - informal imperative
• Alicia compra la camisa. (Alicia buys the shirt.) - present indicative
• Alicia, compra la camisa. (Alicia, buy the shirt.) - informal imperative" from
Ahh I knew context was going to be in there somewhere (LOL) So often that's the answer when dealing with these isolated sentences and phrases. Since there isn't any (and isn't going to BE any) that can be a frustrating response even when it's the right one.
I suspected that the correct punctuation might be the key but in this one there isn't any that helps. We have "Visita alguno de ellos." which DL translates into the informal imperative "Visit one of them."
One of the hardest things for me has been the dropping of the 'who' and going straight to the 'what'. I suppose learning means adjusting but it seems to make so many of the phrases vague. I imagine myself constantly asking "¿A quién te refieres?" when I try to speak Spanish!
"The tour at 3:00 lists three sites."
(It) visits one of them. (but not the other two) - present indicative
Visit one of them. (they include the other two) - informal imperative
The dreaded context strikes again! :)
Thanks for the great answer and the link though.