GroupJive/ Create Group/ Category & Access

I have two questions on creating a new group.

1.On Create Group page, If there are sub-categories under the parent categories, is there a way to show the sub-category selection field afterward the user select the parent category?

2. On Create Group page, under Access field, there are 4 options. I don't know the difference of Everybody vs. Public and All Registered Users vs. Registered. Please see attached screenshot.

Thanks.

1.On Create Group page, If there are sub-categories under the parent categories, is there a way to show the sub-category selection field afterward the user select the parent category?

No a group doesn't have a parent category and sub-category. It only has a single category, which is either the parent category or one of its sub-categories.

2. On Create Group page, under Access field, there are 4 options. I don't know the difference of Everybody vs. Public and All Registered Users vs. Registered. Please see attached screenshot.

They're custom access rules. There isn't a difference depending on how your site is configured. In a future release that will change to view access level instead of usergroup though.

1. Understood that one group belongs to one category.
There are a lot of sub categories in my site, thought users can have difficult time to find the right category from the long dropdown window, not the end of the world though.
Also, I want to force users to select the sub categories, not parent category by mistake.

You can try turning off group creation for the parent category, but that likely will shut off the group creation for the nested categories as well. I'm not sure what to advise for your situation as it was not designed into GJ to function how you're wanting.