Should we just create a global user account and re-register wyBuild or do we need to purchase separate licenses for each developer?
wyBuild is licensed per-developer. If you have 5 developers using wyBuild then you should buy 5 licenses.
Once you've bought licenses for all the developers that will be using wyBuild, you can enter the product key on installation. That is, re-download wyBuild and paste the product key in the installer. wyBuild will now be able to run on all users for that machine.