Presumably, this is a text field with an auto-enter calculation.
Try this version and see if it works for you:
Case ( Entered_By = "Andrew"; "Andrew Jones";
Entered_By = "Greg"; "Greg Smith";
Entered_By = "Kim"; "Kim Greene";
Entered_By = "Bob"; "Bob Johnson";
If it doesn't, please describe exactly how it doesn't work.
What happens when you have two Andrews?
Why not make their User Name their full name, and set in their preferences that as their default name? No additional typing, no Case field needed.
@RW : We do have two Andrews. One is Andy and one is Andrew. Our company is very small, this would never be an issue here.
@Sorbsbuster: I don't want to make their user name their full name because a lot of work has already been invested that relates to their first name being their username. Plus, we don't need to see the full name on our end, just the customer does.
That worked great! Thanks!