In an older version you can still do it from custom code depending on how old it is the method to reset the role cookie may exist on mojoRoleProvider so after adding the user to roles you would call:
mojoRoleProvider.ResetCurrentUserRolesCookie();
It will reset the role cookie for the current user viewing the page and on the next request for a page the new roles will be active. I'm not sure if that method will exist in your version, it has been a long time since we supported VS 2008 and this method may have been added after that I'm just not sure.
In the latest version we have a RolesChanged property on SiteUser so all you do is set it to true and save the user after making any role changes on the user. on the next request by the user the same method will be called automatically to reset the cookie and then RolesChanged will be set back to false.
Hope that helps,
Joe