Anyone know if there is a way to set it up so that LOOKUP function refers to address list/database for one group of users and a different address list/database for another group of users using the same BES

If you're using Exchange, lookup works against the GAL, so I think you're limited to having all users perform lookups against the entire GAL. Not sure about Notes.

Not much help but I'm sure someone else with more experience will chime in, though.