Usually only occurs at parties and events hosted by females. Males who host parties want girls to be there so they buy a lot of alcohol hoping that girls will come and have sex with them. Females on the other hand, host a party for their friends with little amounts of alcohol telling friends to "BYOB" or bring your own bottle/beer. Males come to these parties to socialize with the females and bring their own alcohol in hopes of getting lucky by getting them drunk.
BYOB changes with age. As a child it simply stands for bring your own bed, used mainly during a sleepover to say, "bring a sleepingbag" as you get older, it means bring your own beer,as beer will not be served.
Johnny(age 5) BYOB, friends, it'll be a great sleepover!