A reservation system for travel, accommodation, venue ticketing and other purposes may be accessed on-line, for example via a social networking website. A user enters their social networking identification into the reservation system. When a subsequent user makes a reservation, they also enter their social networking ID and the system retrieves a list of associated parties from the social networking website, which may be a friends or contacts list, and searches for reservations made by any of those parties which have an overlapping criterion, such as date and destination. The overlap may be used to identify parties who will be at the same destination at the same time as the subsequent user, and/or are travelling on the same flight or journey. On check-in, the system may use the list of associated parties to notify the party checking-in that one of their friends or contacts has also checked-in.