Peppermint, I think you may not be aware of the slang phrase 'get loose' but it is a phrase on its own, separated from the phrase 'get lost.' 'Get loose' is a slang way of saying 'have fun'. This comes from the other slang which people say 'loosen up.' When someone is irritated or not enjoying themselves when they should be, people say 'loosen up!' And so from this came the other variation 'get loose!' Which is mostly a thing that is shouted at parties before people take shots and drink alcohol. It is not often said before 'sensible' enjoyments, but rather the crazy kind. So more accurately, it means similar to 'Let's go crazy!'

I think you meant "get lost"..? Loose means "not tight": This is a loose tshirt. = The tshirt is not tight on the body. I haven't heard "Get loose" being used as is. This would be more correct: "you got fat: get those pants loosen up!" = The pants are too tight on you, they should get enlarged. The formulation implies that someone else will work on the pants. Get lost! means "go away!" It is a rather harsh way of saying it. "how did we get lost?" means "we walked so much without paying attention, and now we have no idea of where we are." Hope this helps a little. And I hope others have more and better examples.