What is the ruling on my presence with my tribe in a tribal fight that resulted in the killing of a person, and am I sinful for that, even though I did not participate in the attack?

You bear no sin for the killing, and nothing is required of you concerning it. As for witnessing the tribal gathering, if you knew that it was for a matter forbidden by religious law and you did not denounce it, then you incur sin for sitting with them and increasing their numbers. This is considered an approval of what they gathered for and an assistance to it. Allah Almighty has forbidden cooperation in sin and aggression. Furthermore, one who sits in a gathering of disobedience and does not denounce them is considered complicit in their sin. He should either denounce them or leave the gathering if he is unable to denounce.