Naming Objects in Mongo Array?

Naming Objects in Mongo Array?

Using PHP and Mongo I would like to update the users availability but
cannot figure it out. How can I structure my collection to be able to
reference availability groups.steve.availability?
Below is the structure of my "groups" collection:
{
"_id": {
"$oid": "524327d536b82c7c5c842f6d"
},
"group_id": "testing",
"password": "test",
"users": [
{
"username": "steve",
"availability": "null"
},
{
"username": "joeb",
"availability": "null"
}
]
}