gistfile1.js

const sanityClient = require('@sanity/client')

const client = sanityClient({
  projectId: 'your-project-id',
  dataset: 'production',
  token: 'token-with-create-session-rights',
  useCdn: false
})

const accessDocument = {
  "_id": "_.groups.staff_writer",
  "_type": "system.group",
  "_rev": "wzDKrHcEddnCsDSCdb6KHZ",
  "_createdAt": "2017-12-12T11:46:55Z",
  "_updatedAt": "2017-12-12T11:46:55Z",
  "grants": [
    {
      "filter": "_type == 'categories'",
      "permissions": ["read", "create"]
    },
    {
      "filter": "_type == 'post'",
      "permissions": ["read", "update", "create"]
    }
  ],
  "members": ["e-a-user-id", "e-another-user-id"]
}

client.createOrReplace(accessDocument).then(() => console.log('Access document updated'))