What is wrong with: "numerous men united to a group"? I thought the "a" in "al" indicates a kind of direction "allo zoo" = "to the zoo" etc.
Wouldn't "Numerous men joined the group" be translated into italian by "Numerosi uomini si unirono il gruppo" ?
Italian verbs with a reflexive conjugation are very often intransitive, and unirsi is one of them. This means that any noun standing after this verb acts as an indirect object, therefore between the verb and the noun a preposition is needed (this is why si unirono il gruppo is wrong).
The two prepositions more likely used after unirsi are a ('to') and con ('with'). So you can say:
unirsi al gruppo (preferable)
unirsi con il gruppo or col gruppo
Beware that a, besides indicating a direction, can take many more meanings, depending on the words it is used with, and on the context of the sentence.
In English, if I am not mistaken, the correct preposition after the verb 'to unite' is 'with', rather than 'to' (I hope a native speaker can confirm this), but 'to join the group' would be more common than 'to unite with the group'.
'Several men...' isn't correct? (I mean, instead of 'Numerous men...')
"Numerous men" sounds really odd in English. I haven't found an alternative phrasing that duo will accept, but I will keep trying because "numerous" in this context is so vague as to be meaningless (four? Fifty? A hundred thousand?) which is not helping me discover how to use the word in Italian...