O que significa "Why don't you hop on in there? " ?

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If I had to guess, I’d say ‘hop on’ is the phrasal verb and ‘in there’ is a prepositional phrase.

Hop on is usually used in the context of something that is moving, and you are telling someone to get on. i.e. a merry go round, trampoline, train, bus, etc.

In the context you are describing, it seems that the conversation is the thing that is moving, and someone is inviting you to ‘hop on’ to join the conversation.

Getting ‘in there’ implies going into or inside something. (My brain is imagining going inside a cave, or into the water) I’m this context, it seems implied that the topic is metaphorically ‘deep’, and you are going ‘in there’ or ‘into’ the topic.

Or perhaps the topic isn’t deep, but the way the people are interacting is!