Hi Ken

The way that roles work is that everyone with the same role has the same permissions . If you hide one of the categories to the role then this category will be hidden to everyone assigned this role.

It is possible to enable Audience based visibility and set course visibility to an audience that only contains users in an organisation - this currently isn't able to be applied at a category level.

There are some existing multi-tenant features and quite a few  more being implemented in Totara 13 to separate tenants more.

Regards