Collections aren't used as a proper noun. If you were to say, "Tell them to come here in 5 minutes.", you wouldn't capitalize "them". That being said, I really wouldn't start an email with "Hi, all". There's technically nothing wrong with it, but it feels odd.
Also asked, do you capitalize all words in an email greeting?
If you are using good morning as an email greeting at the beginning of your correspondence, capitalize both words. However, this rule doesn't have anything to do with the phrase “good morning.” It applies because the standard practice is to capitalize the first word and all other nouns in a salutation.
Do you capitalize all the words in a greeting?
Answer: The first word, all nouns, and all titles are capitalized in the salutation. That's according to The Gregg Reference Manual. Gregg says nothing about the last word. As pronouns, all and everyone would not be capitalized unless they were the first word or part of someone's title, according to Gregg.
Do you capitalize all letters in a greeting?
Writing Rules: Capitalizing Salutations and Closings in Letters. Capitalize the first and all major words in the salutation of a letter, but only the first word in the complimentary closing.