Final answer:
The term for an additional musician who temporarily performs or records with a group is a session musician. They are highly skilled and adaptable to different music styles.
Step-by-step explanation:
The term used for an additional musician who is brought in to perform on a specific project and is not normally part of the main recording group is a session musician.
Session musicians are skilled professionals who are hired on a temporary basis to record or perform live with an artist or band.
They may also be referred to at times as a studio musician if they primarily work in a recording studio environment. These musicians are known for their ability to quickly learn new material and adapt their playing style to fit the specific needs of a project.