Create-YammerGroups.ps1

# Yammer app developer token
$token = ""

# Base Yammer API url
$yammerBaseUrl = "https://www.yammer.com/api/v1"

# Reference - Yammer Group 
#$createGroupUri = "https://www.yammer.com/api/v1/groups.json?name=Example&private=True" 

function Get-BaererToken() {
    $headers = @{ Authorization=("Bearer " + $token) }
    return $headers
}

# New group properties
$groupBody = @{ `
    name="New Yammer Group";`
    private=$true;`
    show_in_directory=$false;`
    description="A description about the group."
}

# Post new group
$headers = Get-BaererToken
$groupsApi = "$($yammerBaseUrl)/groups"
Invoke-WebRequest –Uri $groupsApi –Method Post -Headers $headers -Body $groupBody