Complex sentence: Until the British arrived, the iwi was not a governance unit, but among other things, a way to establish kinship and commonality, part of a "who's who" which forms the formal greeting ceremony of "pōwhiri" when one group visits another.
Split into simpler sentences: Until the British arrived, the "iwi" was not a governance unit, but was, among other things, a way to establish kinship and commonality--a kind of "who's who". For example, it is part of the formal greeting ceremony of "pōwhiri" when one group visits another.