Hello Kyle,
I went on applying the hack specifically like you indicated. It still didn't work. I checked the url of the auto action you provided:
index.php?option=com_comprofiler&task=pluginclass&plugin=cbgroupjive&action=groups&func=show&cat=[var1_category]&grp=[var1_id]
I changed the last part of the URL: grp=[var1_id] TO: grp=[var2_id]
This way, it worked!
So for anybody who reads this post, to redirect an user to a group she/he just joined, create an redirect auto action with gj_onAfterJoinGroup as a trigger, and redirect it to:
index.php?option=com_comprofiler&task=pluginclass&plugin=cbgroupjive&action=groups&func=show&cat=[var1_category]&grp=[var2_id]
Thank you for your help Kyle.