Resource Groups

If I assign a Resource to a Project (100%) and add a Resource Group (with the same resource in it) to the same project (again at 100%), the planning takes care not to over allocate a resource to Activities - which is a really neat and useful feature. However, if I add the same Resource to another Resource Group, I get an error "Cannot assign a resource more than 100% across all Pools". I am hoping that I misunderstand the meaning of the rate % in the Resource Group - I thought this is the same as the rate % for the Resource.  I don't understand why I can have a Resource as an individual and in a group - and planning takes care of it, but not in multiple groups.

This is important for me because in a project with few resources at the start, it is better to use resource groups - because sometimes the same resource may temporarily cover multiple roles. Having each role as a resource group helps when additional resources become available - I just add them to the resource group at 100%, and either remove or reduce %to 0 the resource that was covering the role. I don't want or know the % the resource will cover each group - I want the planning to allocate as it would if I had added the resource to each activity.