Validation Rule for a picklist that is limits visibility by Role or Profile
Internal: visibility to "Customer Group" A, B, C, D, E
External D: visibility to "Customer Group" D only
External E: visbility to "Customer Group" E only
I think @mattybme1 might have some Portal experience, so maybe he can chime in.
A validation rule will not limit what people can see - it can throw an error if they pick something you don't want them to, but can't limit visibility.