I’ll be posting the client here publicly on the forums, even if you have the client you can’t join the game unless you’ve “unlocked” Alpha access on your account, so there’s no harm in letting as many people have it as possible.
Unlocking the Alpha access is instant with the new system where you buy the upgrade on your Account page, so even if you (for example) buy the Alpha on Friday after the test has already been going for 24 hours, you would get access right away. So generally speaking from here on out you can buy Alpha access whenever you want and get access right away even if it’s mid-test. I assume we’ll have people doing that as word spreads around that everyone is online having fun haha.
Only thing to note is that the patcher won’t start downloading the actual game files until you’ve unlocked the Alpha access, and the game is around 2GB in size right now, so depending on when you buy the access just keep in mind that you would have to wait for the files to all download to play (so that might take some time depending on your Internet speed).