It depends on the verb. Many people make mistakes on using them. If the verb is direct, use o/a/os/as, if indirect use lhe(s). For a complete explanation, access: http://www.learningportuguese.co.uk/language/object-pronouns.html
You're right. "Lhes" replaces "to them" (a eles) and "to you" (plural) (a vocês).
Anyway, I'm reporting this question because it's grammatically wrong. Some verbs do not admit the pronoun "lhes", such as to assist (assitir) (in the sense of being present), to help (ajudar), to aspire (aspirar), to preside (presidir) and to resort (recorrer), Thus, the right form would be:
Eu ajudarei a você/a eles (not "eu lhes ajudarei"). There are some more rules, but I think it would make it harder for you.
I agree with you. Ajudar takes direct object, the correct is "Ela os ajuda"
But being direct, don't use preposition: "Ela ajuda vocês"
Preposition is used with "ajuda" only when it's a noun, object of another verb:
"Ela envia ajuda a vocês" (She sends help to you)
......but then I went researching, and some say it can take indirect objects as well....damn verb....