When can one use the sentence ender ぞ? I've only ever heard it anime, so I'm unsure of it's actual usage in the real world. Is it not used that often or limited to specific age/gender groups?
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なあ can usually substitute for ぞ in the third category. (This is covered in the same section as ぞ in the abovelinked Google Books preview.) Note that when using ぞ in the second category, ぞ follows the dictionary form of the verb. This is in contrast to ぜ, which often follows the volitional form when the intent is to get someone to participate in an action with you:
ぞ never follows the polite form, and is only used toward friends and persons of lower status.
As far as I noticed, many people use
and it is used like 行こう/行きましょう（意向形）with a bit stronger sense when talking to group of people, mainly if speaker is kind of coordinator / leader.
Yes, it is a very emphatic sentence-ending particle. Also ぜ. They seems stronger than よ (in my opinion). I think they (ぞ／ぜ) are very informal though, so where you could use よ for emphasis in a polite and/or formal way, you probably shouldn't use these.
I think their usage must be a regional or demographic thing. When I lived in Osaka, I never heard it at all, and sometimes another gaijin friend of mine would use it around our Japanese friends (mostly young people in their 20's), they would always give an amused laugh. Maybe I'm over-generalizing it because of the somewhat limited group I was around, but it doesn't seem to be a Kansai thing. Maybe other places use it more commonly?