I managed to resolve this and answer my own questions.
I had to create a BRFplus Flat Rule for the agent rule. This generated the decision table in which I then referenced a DB Lookup expression to read the user id linked to the user group.
As for the errors in IMG configuration, I discovered that I had to use transaction GRNFW_CONFIGURE to do the customizing in expert mode and here I found that even though I had successfully added the new agent rule to the configuration, and saved it there on GRFNW_CONFIGURE_MW, it wasn't saved! Somehow it had disappeared when I used expert mode. So I added it again and was then able to activate the configuration version.
Thanks to anyone who was giving this any thought on my behalf.