Learn English – “Where do you stay?” vs “Where do you live?”

Wikipedia has an article on hella:

Hella is a word associated with Northern California used throughout the United States and Canada. It is a contraction of the phrase "hell of a" or "hell of a lot [of]". It often appears in place of the words "really," "a lot," "totally," "very," and in some cases, "yes." Whereas hell of a is generally used with a noun, according to linguist Pamela Munro, hella is primarily used to modify an adjective such as "good."

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While intensifiers similar to hella exist in many colloquial varieties, hella is unique in its flexibility. It can be used to modify almost any part of speech, as shown below.
That pizza was hella good: hella modifies the adjective good, where Standard American English would use very.
I ate hella pizza: hella modifies the noun pizza, replacing a lot of.
I hella bought four pizzas: hella modifies the verb to buy, replacing really or totally.
I ran hella quickly to the pizza joint when I heard about the one dollar cheese slices from my friend when we were talking at four in the afternoon one Saturday night: hella modifies the adverb quickly, replacing very.

This does at least answer your second question (whether there were any other usages for this word that you were missing).

Judging from what I've heard of the word, I would say that your third example usage is pretty uncommon. A more normal usage would be:

You have to walk hella far to get to that subway station.

A crude proof would be to Google "walk hella"; the results show walk hella far, walk hella slow, etc. There are no examples of walk hella.

When you use the word group your emphasis should be on the collection of things in the group.

How many people do you have in your group?

When you use the word grouping your emphasis should be on the act of forming the group rather than the group itself.

Which grouping would be better- girls in one group and guys in another, or else adults vs children?