I haven't read these thoroughly, but hopefully it's a place to start. I searched for "picking victims study." That seemed to come up with some hits. A search for "group dynamics abuse" was less helpful.

Targets who displayed vulnerable body language were more likely to report past histories of victimization, and psychopaths identified these individuals as being more vulnerable to future victimization.

https://www.workplacebullying.org/gait/

What Do Sexual Predators Look For When Grooming Victims?

https://www.oxygen.com/crime-time/jeffrey-epstein-what-do-se...

Short Circuiting the Victim Selection Process

http://www.protectivestrategies.com/victim-selection.html

Marked for Mayhem

https://www.psychologytoday.com/us/articles/200901/marked-ma...