Multi-perpetrator sexual assault, more commonly called gang rape, can take a number of different forms.
It can be planned out ahead of time by the perpetrators, with or without the use of drugs on the victim, or it can be used as a form of initiation into a group like gang, fraternity, club, or sports team.
You might also encounter sexually harassing behaviors outside work or school.
Sexual Harassment in the Workplace Defined - css-mania.info
Defining the different types of sexual assault -- Survivor Alliance
Forms of Sexual Harassment
Legal definitions vary by state, but this typically means relatives that are first cousins or closer in relationship.