I just made a free Breast Expansion Archive account. I had to mull the decision over for a few days before I took the plunge, though.
See, signing up for an online service is a risk: Every bit of personal information you give away could come back to bite you (most likely in the form of spam). And porn-focused sites... Well, they don't exactly have a sterling reputation. The Breast Expansion Archive's "Privacy Policy" page does says that the BEA isn't partnered with any ad serving companies... but it doesn't guarantee things will stay that way, so I was a bit worried.
Still, I figured that the quantity and quality of stories in the Addventure is definitely worth the risks inherent in signing up for a new online service.
The only reason I was able to make that judgement call, though, is that I've been visiting and reading the Addventure for over a decade now. I've learned what to expect; I know how good it is, how much content there is in the archives, and how easy it is to add new episodes of my own. I knew exactly what I was signing up for.
But for someone unfamiliar with the Addventure, think about what their first-time user experience looks like:
1. They find a link to an Addventure episode somewhere on the internet. On a forum, perhaps. The text surrounding the link promises an amusing story relevant to whatever was being discussed in the thread.
2. They take a leap of faith, and click on the link.
3. They immediately find themselves on the sign-up page for a porn site. This page makes no reference to the "Addventure episode" they wanted to see. From there, there's a few different ways they could react.
4A. They (incorrectly but understandably) assume that the page they're on is not related to the story they were promised, and that the forum post that sent them there was either trolling or spam. They hit the back button, report the forum post, and ignore similar-looking links in future.
4B. They (correctly) assume that the page they need to go to is hidden behind a membership wall. However, since the sign-up page doesn't contain even a sample of what they came to see, they have no reason to think it'll be interesting enough to be worth signing up to porn site. They hit the back button.
4C. They (correctly) assume that the page they need to go to is hidden behind a membership wall and are curious enough to sign up for a free account. However, signing up takes them through a number of different screens; Once they've finished, there's no easy way for them to navigate back to the content they wanted to find.
None of those outcomes are particularly good. Even 4C, which gives the BE Archive a new user, gives that user a bad first-time user experience; The other two actively discourage people who aren't already BE Archive members from reading or contributing episodes.
I can understand why making the Addventure require a log-in might seem like a good idea. After all, the new site's stated philosophy is to break down the distinction between content creators and consumers, and the Addventure is that incarnate. Besides, requiring Addventure users to sign up for a BE Archive account will cause a huge upsurge in the number of new BE Archive accounts being created as existing Addventure users sign up.
That's a short-term benefit, however. In the long term, fewer new users will start reading the Addventure, and that means fewer people will become Addventure authors. This means that the number of authors and readers will decline over time - and as others have already said, discouraging new people from becoming users is a big risk for a service that depends on crowdsourcing for new content.
There's also a risk of losing your existing users, including the authors who make the Addventure worth visiting. I'm still here, obviously, but there are a lot of Addventure authors out there. As others in this thread have already mentioned, those who click on their bookmarks and see the new sign-up page won't necessarily realise that it means the Addventure is now members-only: Some may assume that it's down temporarily as a result of your site upgrades, or that it's gone forever, or that it now requires a paid membership. Of those who do realise that the Addventure is still available, some of them may decide that they don't want to sign up - and that means that you're losing authors who would otherwise help attract people to your site.
tl;dr: Putting the Addventure behind a membership wall is causing problems.
I have some unsolicited suggestions that I (as an unskilled commenter with no financial stake in your company) imagine might help mitigate these problems:
1. Make it possible to read the Addventure without having to sign in. Some of it, at least. Perhaps make episodes readable to anyone, but hide their options, or something. Unless you actually demonstrate to potential members what the Addventure is prior to their signing up, there's no way for it to act as an incentive for them to join.
2. If you definitely want to keep the Addventure behind a membership wall, and don't want to lose the existing Addventure readers and authors, put a line on your sign-up page that says "Sign up for access to the Unending BE Addventure, our image galleries, and other content." I know it's in the FAQ, but as other posters have mentioned, people might assume the site's just down and not get as far as reading the FAQ page.
3. If you want the Addventure to funnel its users to your paid content, put a link to the Breast Expansion Archive into each BE Addventure episode. Even a simple text link under "Go back" would do a lot to drive traffic. (Currently it contains very few links to the Breast Expansion Archive, and hardly ever mentions the BEA by name. It was previously possible to use the Addventure without realising the BE Archive existed, so I wouldn't be surprised if it wasn't driving many people to sign up.)