Guinan was an El-Aurian. Her race (including her) was rescued by the Enterprise-B at the beginning of ST:Generations from the Nexus. From then on the remaining El-Aurians were linked by the Nexus because they existed in both within the Nexus and outside of the Nexus. This was referred to in at least one episode of ST:NG by both Q and Guinan. This would explain why she named the bar Ten Forward and her longevity.