Are most angel investors investing as individuals or as part of an angel "group"?

There are many angel investors in both categories. Here is a brief overview of both:

Organized Groups:
Here in Silicon Valley, for example, there are several groups including Band of Angels, Sand Hill Angels, The Angels Forum, Harvard Angels and Keirestu Forum, among others. Typically with these groups, the investment decision is made at the group level, yet even still each member decides individually whether to participate in the round. All the investors from the group share a Term Sheet for the round. Although it is difficult to generalize, these groups are typically looking to fund about $250K-$500K in the first seed round of a startup.
In my case, I am a member of the Band of Angels, and if you have questions about the mechanics of organized Angel groups, I could answer with specific examples from our group, if that would be of help.

Individual Angel investors:
These come in many flavors. They can simply be high net-worth individuals (specifically, “Accredited Investors,” http://www.sec.gov/answers/accred.htm) who invest in friend’s and colleague’s startups as a sideline interest. They can be full-time investors, who are typically very highly networked individuals who are very involved in many companies, at a board level. And at the high end is the emerging class of so-called “Super Angels.” These Angel were generally so successful as individual Angels that they have also raised modest funds ($5M-30M range) to compliment their own investments. In Silicon Valley, prominent examples include Ron Conway’s SVAngel fund, Mike Maples’ Floodgate fund, Dave McClure’s 500 Startups fund, and Jeff Clavier’s Softech VC fund.