Firewall using workgroup

A company previously built without a firewall and using a workgroup is in the process of expanding to 100 employees and is against any major expense or changes. How would you argue the necessity of a Domain setup and adding a firewall? If the users will rotate machines and multiple people will be using each individual computer at different times, what are the major advantages to using a Domain controlled network? What are some of the concerns and disadvantages to switching to a domain network?