A group of individuals that may not have a biological relationship with each other. For example plantations, orphanages, military units, etc.
"groups": [
"id": "GR001",
"individuals": [
{ "id": "IN001" }
{ "id": "http://williamsfamily.example.com/tree/IN002" }
{...}
],
"desc": "Peers",
"date": [...],
"place": {...}
]