Public Enemy Number 1 (PENI) has grown steadily since the early 2000s. In 2003, the membership was estimated by law enforcement to be about 200; by 2005, the group had grown to 350-400 members documented by prison officials, although a substantial number of members and associates are unknown to authorities.  Its total membership, including associates, could be as much as twice its documented membership.

PENI's growing clout in the prison system and control over criminal activities makes it attractive to some white inmates from the general population who seek protection from ethnic and criminal gangs and other prisoners. Some members of white supremacist street gangs join PENI while incarcerated, then resume an association with their own street gang after their release.  Due to a shared ideology, PENI members can easily blend in with other white racist gangs. This kind of dual membership makes it difficult for law enforcement to assess PENI's size or attribute crimes to its members.  However, because PENI members often "tier up," or cluster around one particular residence, which then becomes their main gathering place, it is easier for law enforcement to detect the group's presence in certain areas.

Individual PENI members have been documented throughout the country and even abroad.  However, according to the California Department of Justice, approximately 300 of the group's documented members operate in southern and northern California.  They appear to be most heavily concentrated in Orange County, where they are active in almost every city of the county, particularly in Anaheim, Costa Mesa, Garden Grove, Huntington Beach and South Orange County.  They also have a strong presence in the Inland Empire, especially in Riverside and San Bernardino and exist in smaller numbers in San Diego and Los Angeles counties.

PENI is less active in northern California, where it lacks strength in numbers, but according to law enforcement, it does have a presence in Sacramento, Redding and Shasta.  More recently, the group has begun to recruit in Arizona, near Lake Havasu and Bullhead City.  There are as of yet only a handful of documented PENI members in the Arizona prison system, however.