Marriage between more than two people

If more than two people are involved, what happens when one of the parties leaves the relationship, or dies, or when another party joins it?

In an unofficial arrangement, the parties can make whatever agreement they like between all of them. They can call it marriage, if they like.
But if the arrangement is made a legal one, you have to consider property rights, inheritance rights, who is next of kin, arrangements for the support of any children etc. These rights and responsibilities have to be spelled out in laws.

Marriage and divorce laws are already complicated enough. Laws to cover three, four or a dozen legally married partners would be a beanfeast for the lawyers.

Trying to accommodate families from cultures where a man is allowed more than one wife is complicated enough.

But suppose a legal marriage involved several men and several women?

Chaos!