As far as I understand you want to hide some sections of your website to users. Then you want to give access to this sections for some users. You can use membership plugins which has ability to make membership types. Like at Membership 2 Pro plugin you can make membership types. For example; normal user type, beta user type. You can use for limit pages, posts, categories, videos, audio, images, PDFs, digital downloads, forums, menus. But theme and plugin is another layer I think and not support preventing plugin works for spesific user. That need custom coding I think.

I don’t know there is free alternative to Membership 2 Pro but you can give it a chance.

I do not think S2Member is an option (though I asked there because I own it), because it basically would mean that the people are signed up after they entered the code. Same, i guess, with Membership 2 Pro. I pretty much only would like a check if the code is valid and then have a cookie set to that the splash page is gone for now and for future visits.

My people should be needed to sing up the normal way, because this needs to be part of the testing process and because the signup seperates different user groups etc.

Marian