According to HowStuffWorks.com, most barnacle species reproduce with nearby members of their collection through the use of male and female sex organs, or by themselves if there are no other barnacles nearby. The majority of barnacle species are hermaphroditic, which means that they do not necessarily need to mate with one another in order to reproduce.
Wikipedia explains that most barnacle species contain both male and female reproductive organs and are thereby able to produce eggs as well as the sperm necessary to fertilize their own eggs in order to produce offspring. This form of reproduction among barnacles is considered very rare, however.
It is most common for adult barnacles to collect and arrange themselves in permanently attached clusters on underwater items, such as rocks, wharfs, ships and even other marine animals. When barnacles are situated near one another during mating season, males with functional penises reach out of their crustaceous pods and seek to inseminate functional, fertile egg-carrying females nearby. Marine biologists believe that barnacles have the largest penises in the animal kingdom relative to their body size. If barnacles are not located closely enough to one another, males may release their sperm into the surrounding waters, allowing an opportunity for females to reach out of their pods and obtain the sperm by pulling it into their uteri.