Back in the old days, we always used to say no, you can’t enforce group rules. Feel free to make up the rule and enforce it within your group, but when it comes to out of the group, it’s not like it can be enforced on a staff verdict. Everything’s case by case these days, I would say if the particular player was in the group, he signed the unofficial rule and then died consequence of the group rules, the staff team would look into how it played out RP wise and whether it was justified from outside of the rules. If there was a fair bit of story behind it and everyone’s still in the group, then I’d personally say it’s shame on them for signing their unofficial rule. If they joined the group and then the next day they were gunned down without proper IC cause, then it’d be treated as a normal KOS.

If the staff team treat it differently to the above and just say no full stop, then I’d suggest removing the whole “unofficial group rules” thing.