You should be able to prevent access by using Joomla menu items. So for example set category access to Public, but set the menu access to reach GJ to Registered. What category access were you using? It's possible an improvement to the auto join field behavior would be the best solution. For example I can have it check category access based off the usergroup they're about to have once they become registered, which would be based off the new user registration usergroup parameter.