Are you using a domain called WORKGROUP, or do you mean they both PC's are in the Workgroup WORKGROUP?

If the latter, I've had no experience with using SSRS using Workgroups. You should, however, still be able to add the users of the other PC to the permissions; you'll just need to specify the PC name as part of the username. For example PCName\Thom rather than just Thom. You'll need to add their permissions first by connecting locally.

Edit: Ok, seems, after a quick Google (a wondering tool, you should try it 😉 ) that you can't authenticate via a Workgroup. You'll need to disable anonymous authentication on IIS, and then any users accessing the SSRS instance from a different machine will need to use credentials of a User on the Machine SSRS is hosted.