As the subject, i want to allow frontend user to add their own extra fields according to their own needs while they are submitting the article. Any ideas?

I'm wondering the same thing.

I am not 100% but i think the creation of extra fields, extra field groups, and assignment of the same is limited to the design stages and the administrator interface. Even if you give users limited access to the backend they will most likely break things.

I was thinking something like a button with ADD FIELD and a field will appear under the last field.. or something like this.

That would require backend access and privilege as well I am afraid.