I don't think you can pin down a single "point" where it suddenly becomes better to use a domain vs. a workgroup.
That said, I generally consider around 5 computers to be the max size for a workgroup. Any larger than that, and domains start to have some attractive advantages that make it worth the effort.
Some criteria that might suggest a domain is the right choice:
- You need a central location for sharing files/enforcing permissions
- You have users that change passwords or even users that come and go (e.g., new employees)
- You have users that aren't good at "managing" their own computers (i.e., they don't install updates, don't know not to run suspicious attachments, etc.) and so need the "structure" and security that you can get from a domain
- Windows Update Services is really nice once you get it up & running, and ensures that computers are up-to-date.
- You need (or want) to use some of the features that a domain's Group Policy can give you
There are some factors that make a domain impossible of course:
- You have a bunch of "home" type computers (e.g., running "Home" versions of Windows and therefore can't join a domain). (You could replace the OS of course, but I'm assuming that if you're using "home" machines, you might not have access to or the budget for the "pro" versions of the OS)
- You don't have a server
- The computers aren't yours (you don't have permission to join them to a domain; as in say people who bring in their own laptops and don't want to join a domain that they won't be able to access while at home)
Like so many things, the choice of whether to set up a simple workgroup or a domain is a very personal, subjective one... anyway, these are some of the things that guide my decision when I have to make that choice. Hopefully they will help you as well.