Community-Based Gang Intervention Act This bill extends federal support for juvenile justice and delinquency prevention efforts to programs and activities involving community-based gang intervention. First, the bill authorizes community-based gang intervention grants for nonprofit community-based organizations to reduce and stop gang-related and gang-motivated violence and crime. Second, the bill modifies the qualifications of representatives of state advisory groups to specify that (1) a representative of a public agency includes an individual from a public agency concerned with community-based gang intervention, and (2) a representative of a nonprofit organization includes a person with a special focus on community-based gang intervention. Third, the bill adds, as a purpose area under the Incentive Youth Promise Grants for Local Delinquency Prevention Programs, providing community-based gang intervention and gang prevention activities to juveniles and their families.