The 2009 U.S. Department of Justice study (link below) states in Table 3 (page 12) in the appendix that 13.1% of the roughly 3.4 million stalking victims in America has three or more stalkers which equates to roughly 445,000 victims with 3 or more stalkers. So, you can see that this group stalking is a real phenomenon and comes from a credible source.
http://bjs.ojp.usdoj.gov/content/pub/pdf/svus.pdf
There has been more information released about the break down of the statistics cited above in that 185,000 respondents claim a group of three or more are working together to stalk them. Use the link below.
http://www.nowpublic.com/world/gang-stalking-new-doj-foia-documents-prove-doj-knows-truth