Aside from official ranks such as officers and raid leaders, there are plenty of niche roles that guild members can fall into. I'm personally of the opinion that every guild can benefit from having a designated whipping boy to call their own. You know who I'm talking about. Everyone cracks jokes at his expense. Failures are blamed on him, even when he's not logged on. From the outsider's viewpoint, it might look suspiciously like everybody hates him! Yeah, that guy.
However, it's all done with tongues planted firmly in cheeks, and everybody has nothin' but love for their friendly neighborhood scapegoat. In reality, he is not just some pusillanimous pushover ... He is simply that special breed of person strong enough to take all that good-natured ribbing for what it really is. (Of course, if my guild's own whipping boy ever stumbles upon this, I'll completely deny ever writing such nice things about him.)
Then there's the Chief of the Guild Note Police. This handy person is obsessed with guild notes and will make sure that they are always filled with useful information. When a new member joins the guild, the Chief is the first to bluntly ask, "Hey, who are you?" Without their dedication, you might not know that the level 12 worgen who just logged on is actually your guild leader.
These are just a few of the various personalities I've seen during my time in Azeroth. What kinds of minor roles have you noticed folks playing in your guild? Is there a specific type of member you believe no guild should be without? What about you? Is there a role you tend to play amongst your circle of friends?