A lengthy new post on the site homepage explains the recent changes:

Good news, everyone!

After lengthy discussions, we've decided to reverse ourselves on the position that we recently established, regarding how hard it's going to be to get into ScienceHD. We initially decided to go into lockdown in large part due two reasons: the abuses of many of our users, and security risks. Our ultimate goal is the sharing of knowledge, though, so this was a very hard choice to make, and we're very glad to be able to open up again. We've been hard at work behind the scenes, implementing new security features and reorganized things, and these new changes have alleviated many of our previous concerns regarding security and bad users, among other things.

Now, ScienceHD won't be fully open registration, though. Here is how things will now stand:

There will be a questionnaire, open to all interested parties, to sign up with. Unlike our Gravmas event, this invite questionnaire will always be open. It will be reviewed manually by staff to decide whether an invitation will be extended.

All registration and invite-sending will be subject to the Max User limit, although this will be raised and kept at a generally high level. No invites or registration will be allowed if the number of enabled users exceeds this value, with the exception that:

PhD's and above will be able to send invites at any time.

However, invites will remain fairly hard to get. They'll be given out for competition rewards, for notable contributions to the site, and possibly available for purchase at a high price in the future.

Now go forth, and multiply!