FunctionsPublic/Add-GraphMemberToGroup.ps1

function Add-GraphMemberToGroup
{
    param(
        [psobject]$accessToken,
        [string]$groupID, 
        [string]$userID
    )
    
    $sendBody = @{
        "@odata.id"="https://graph.microsoft.com/v1.0/users/$($userID)"
        } | ConvertTo-Json 
    
    
    $responseBody = Invoke-RestMethod `
        -Uri "https://graph.microsoft.com/v1.0/groups/$($groupID)/members/`$ref" `
        -Headers @{"Authorization" = "Bearer $($accessToken.AccessTokenCredential.GetNetworkCredential().password)"} `
        -Body $sendBody `
        -ContentType "application/json" `
        -Method POST

    $jsonResponse = $responseBody | ConvertTo-JSON

    $addOwnerResult = ConvertFrom-Json -InputObject $jsonResponse

    return $addOwnerResult
}