Linda:
Check this:
RoleController.AddUserRole(PortalID, UserID, RoleID, ExpiryDate)
Edit: You can also do this to use the role name if you don't know the RoleID:
RoleController.AddUserRole(PortalID, UserID, RoleController.GetRoleByName(PortalID, "RoleNameString").RoleID, ExpiryDate)
I think this is the more "proper" way.
Carlos