The Summer Tree

Five men and women find themselves flung into the magical land of Fionavar, First of all Worlds. They have been called there by the mage Loren Silvercloak, and quickly find themselves drawn into the complex tapestry of events. For Kim, Paul, Kevin, Jennifer and Dave all have their own part to play in the coming battle against the forces of evil led by the fallen god Rakoth Maugrim and his dark hordes.

First volume in series.

Synopsis from Open Library.