The site appears to be fucked up, sadly.
Here's how it should work:
Go to
Grupos versão 5.0 :: Pictures REDDNAT
Click on the yellow button "assine o grupo" (in the left-hand column) to sign up.
Enter your email address and (alongside 'apelido') the user name you'd like; then click on 'assinar'.
Go to your email a/c and click on the link in the mail from the site, under the phrase: "Para isso, você deve acessar o seguinte endereço:"...
..which takes you to the site, and will tell you that you're a confirmed member.
And then it asks you to log in, using your ID and password (senha).
But you don't have a password. 
You weren't asked for one when signing up; there wasn't one provided in the email from the site; and the second email you received after confirming your email address... doesn't contain one either.
So you try to get your 'lost' password (
so to speak) sent to you by clicking on: "Esqueci minha senha". It asks you for your email address. No matter what email address you enter that you already signed up with (and I've tried it with
3 different addresses that the site had already 'confirmed'!), it tells you that the email address is 'incorrect'.
At which point, you wonder whether looking at the LPSG gallery wasn't a better option in the first place.
That's my experience (confirmed again today), anyway. 