Try this
public string CreateUser(Registration reg)
{
reg.Token = Guid.NewGuid().ToString();
Session.Store(reg);
Session.SaveChanges();
AddUserToRoles("user") //it depends here whether u want to pass role id or role name then create the overloaded function Accordingly
return reg.Token;
}
create overrided for AddUserToRole
public string AddUserToRoles(string role)
{
//assign user to role.
}