Pretty sure a comma is correct in these. Thanks, all. Hi, everyone. Thanks, everybody. But no comma, I say, in these: Hi there. Hello there. Agreed?
I think I would agree with the second batch on the grounds that "there" is technically an adverbial phrase, and it's at the end of the utterance, where adverbs naturally belong. In the first examples, you clearly have a vocative/interjection, which should be set off by commas.
