if i cause 100 damage with a skill in a particular enemy, and after that use join penalty, linking the enemy with 4 others opponents (total 5), and I cause again 100 damage in one of them, all others will suffer 100 damage or each suffer 20 damage (100/5)?
Yep, they will share all the dmg normally, 100% hit for all.
http://www.tosbase.com/content/video/skills/Linker_Joint_Penalty.mp4