Hi Krileon,
Can you see all the thread or only this post?
Back to my first post: the question is "Is it possible to send an e-mail to the user who has joined an approval type group when the owner of the group makes the approval?"

You told me: "You might be able to using CB Auto Actions with the gj_onAfterUpdateUserStatus trigger."

I upgraded to CB 1.8. - GJ 2.5.1 and CB Auto Actions 2.4.2 .
I created a new Auto Action:
Type: Email
Triggers: gj_onAfterUpdateUserStatus
User: Automatic
Access: All Registered Users
To: [email]
Subject: Mysubject
Body: Mybody
Mode: HTML

But it doesn't work.

The plugins order is correct. I tried some combinations and finally it worked! The problem was the "Access" parameter. The auto action only worked with "Everybody". I tried with "All Registered Users", "Registered" and "All Registered User"+"All Moderators" but they don't work. Only "Everybody" works.
Solved but, any idea why?

Thanks, Krileon.

Yes, that works, forcing the user variable as it's Variable 5, setting User parameter to "All Registered User" and the email is sent. But there is another problem. I wrote a fixed email address (without substitutions) as the "To" parameter in the auto action and it works, but if I try to use [email] substitution it doesn't work as I expected. That's what I found when I put these substitutions in the "Body" parameter:
Var 1: [var1_email]
Var 2: [var2_email]
Var 3: [var3_email]
Var 4: [var4_email]
Var 5: ronvix@mixmail.com (the moderator address)
Var 6: [var6_email]

As you can see, only var5 email substitution works but I get the moderator data. So I can not send the e-mail to the user that has been approved. If I choose "User" as the "User" parameter, I get moderator data too for [email] substitution.
Any idea about how to obtain the approved user e-mail?