The volunteer part refers to the fact that while you have to be invited to this group, you are a volunteer in the sense that it is an unpaid "position". You receive same extra tools to help fight spam and vandalism across fandom, but you are not part of the company, so you don't get paid, you don't have office hours, you don't have deadlines, etc. You do what you can when you can.
There's a better explanation, thank you Lady Lostris. However in my opinion, the misleading part of that is the distinction between using "volunteer" as either a noun or a verb. At first glance, it looks as though anyone can "volunteer" to be a part of the group, but I digress, I can be a bit nit-picky