I still can't get it to work (and the fact that Google insists on using Swedish for me makes everything ten times harder to google)...
Where is that? I have added the testers to (and now I guess the English names of things, as, as mentioned earlier, I can only get it in Swedish) 'Settings' -> 'Account information', in the box 'Gmail accounts with access to test functions'. I've added both the mail account of the tester and the googlegroups.com account.
No link appears below, and, from there on, of course, I can't follow the instructions.
Edit: I can see the tester account under 'User accounts and permissions', and it looks OK.