LinuxQuestions.org - Pure-ftpd server rusers?

I have an anonymous pure-ftpd server running in one computer, so when a person connects to the FTP server, what user does he becomes in the server computer? I mean, how the computer server knows that FTP clients only can access to the FTP folder? I think it is because there is a group in the server computer called ftp and when someone connects by the FTP ptotocol he becomes a member of this group? But what is his username? How can I see whats user are connected at each moment? How can I identify them? Maybe you cannot know who they are because they are anonymous, but in the server computer they must have some ID.

I have explained as better as possible, I hope all of you can understand it. Sorry for my English :p