An arranger is a person capable of preparing and adapting an already written composition for presentation in other than its original form. Arranging is the art of giving an existing melody musical variety. The tag can be used for those persons that do so.
From wikipedia: The American Federation of Musicians defines arranging as "the art of preparing and adapting an already written composition for presentation in other than its original form. An arrangement may include reharmonization, paraphrasing, and/or development of a composition, so that it fully represents the melodic, harmonic, and rhythmic structure". Orchestration differs in that it is only adapting music for an orchestra or musical ensemble while arranging "involves adding compositional techniques, such as new thematic material for introductions, transitions, or modulations, and endings... .
