Automated Role Creation

i seem to have some misunderstanding about how roles work in sql server
2005. i see that i can add a role to a database
(dbname->[right-click]->properties->permissions->[add ...]. THis allows
me to add either users or roles. Users can be added programmatically
using sp_grantdbaccess @username, but this does not allow for addition
of roles to access the database (i.e., sp_grantdbaccess @rolename does
not work).

Is there some other command that is used to add a role to the
database's permissions list? Seems there must be since all the other
parts of the permissions chain work quite well:
* sp_addrole
* sp_addrolemember
* grant <permissionon role::@rolename
etc.

So the missing ingredient of something like sp_grantdbroleaccess is
what I need and can't seem to find it anywhere!

Thanks much for any help