It just so happens that SMS are polling the community on whether they (SMS) will reduce their stake and in order to allow some new members to sign up. The Yes's seem to be winning. Keep your eyes peeled as you might have a chance to sign up in the near future.
if you already have an account, can you still buy?
I created an account a while ago, but never got around to buying into it because I didn't know if my graphics card would be up to it as it was a dual GPU card.
If it is a free user account, then at the moment you cannot buy a membership. But if the poll that is being conducted at the moment produces a yes vote (looking likely) then you will be able to buy in.
Probably within a week. Presuming it is a Yes vote then I am not sure how soon after that they will start taking new members. I do believe that it will be a limited time period though. A weekend or so.
Your best bet is to get yourself a free user account so you can read the forums.