- Koby Abberton is the leader of the gang with a reputation for violence. He was jailed for assault. You fail to make the case that this is not relevant. Just because there is a separate article for him, there is no reason that content cannot overlap in two different, but linked, articles. Check out the article on the Red Army Faction/Baader Meinhof Gang. In it, there is a discussion of one of group's leaders Horst Mahler, who has joined the far right and became a militant antisemite and Holocaust denier. This is both interesting and relevant (and I see that you like factoids). But in your line of argument, this would have no place in the article because it would belong solely in the article about Mahler. This is not an isolated example of a leader's actions/thinking being discussed in an article about a group.  

 - Discuss: to examine by argument; to sift the considerations for or against; to expand on a subject. We are doing the latter. This is not an attempt at original research, we are hopefully making this piece a little more comprehensive and balanced. 

 - "Activities of person (a person, persons, people?) who have (an) affiliation with the group that are not able to be presented in (the) context of (the) group itself are not relevant here." Koby Abberton assaulted an off-duty police officer in Hawaii and was jailed for doing so. The Bra Boys came to public attention after a brawl between them and a group of off-duty police officers. Koby doesn't just have an "affiliation with the group" he and his brother are the leaders and the most famous members of it. Explain how it's not relevant?