Sorin Popa initiated the study of Polish groups which are embeddable into the unitary group of a separable finite von Neumann algebra. Such groups are called of finite type or said to belong to the class U-fin. We give necessary and sufficient conditions for Polish groups to be of finite type, and construct examples of such groups for I-infinity and II infinity von Neumann algebras. We also discuss permanence properties of finite type groups under various algebraic operations. We close the paper with some questions concerning such groups.