Preformed group is a Tour group in existence prior to the tour, the members of which share a common bond, interest or organizational affiliation. Also referred to as affinity group.
Other definition:
Preformed group is a pre-existing collection of travelers, such as affinity groups and
travel clubs, whose members share a common interest or organizational affiliation.
The Travel-Glossary: Everything about Travelling, Sightseeing and the World.