An unassailable group seems impossible given that there shitty people out there, who if they join such a group, immediately mean that someone is not necessarily trustworthy because of merely being a part of said group. Even a belief that one’s group is an unassailable paragon seems problematic as if one truly thinks that one’s group is unassailable, then any accusation of wrongdoing by an outsider towards a member will get dismissed, and you could get a situation like some religious groups get with priests or others that they see as inherently good and trustworthy, where when an abusive person inevitably attains that status, allegations against them are dismissed and covered up.