In addition to your 'users' table, create tables for each type of role. These can contain attributes specific to that type of user. For instance, you can have a admin type of user with specific flags allowing different types of CRUD access. A business contact user might have additional ways to be contacted (phone extension, cell phone, mail stop, etc.) For each table, add a user_id foreign key. In the models, make sure these all 'belongs_to :user'.
Then in the User model, add a 'has_one :admin' and 'has_one :business_contact' etc. for each associated table. The has_one association adds methods to the model to make it easy to query the relation:
user = User.find 1
# do admin stuff
details.save unless user.business_contact.nil?
Each user can participate in more than one role. This keeps the user login centralized and the authorization stuff very readable. Adding a new type only requires adding a new table and updating app/models/user.rb to add the has_one associtation.
This simple strategy might not be enough for your needs. It doesn't work if you anticipate adding user roles on the fly to a running system. In this case, you'd probably want to look into a permissions system tied to a HABTM based Role and UserRole model. I've only described a solution for a fixed amount of user types, more or less hard coded into the models.