I don't understand this category of badges...

I mistakenly thought that these five badges were in the same category. But lately, I began to wonder if these badges require the same action - and what that action truly is.

The badges are:





And then the oddball:


All five badges explicitly say that I have to become a member of Khan Academy (and in most cases stay in that position).

But for each badge, what does it mean to "become a member of Khan Academy"? Are these badges all in the same category?