Create the default groups via PowerShell

If your site is missing the three default groups “Visitors”, “Members” and “Owners”, you can create them easily with PowerShell or the Object Model.

Add-PSSnapin Microsoft.SharePoint.PowerShell

$web = Get-SPWeb
if ($web.AssociatedVisitorGroup -eq $null) {
    Write-Host 'The Visitor Group does not exist. It will be created...' -ForegroundColor DarkYellow
    $currentLogin = $web.CurrentUser.LoginName

    if ($web.CurrentUser.IsSiteAdmin -eq $false){
        Write-Host ('The user '+$currentLogin+' needs to be a SiteCollection administrator, to create the default groups.') -ForegroundColor Red

    $web.CreateDefaultAssociatedGroups($currentLogin, $currentLogin, [System.String].Empty)
    Write-Host 'The default Groups have been created.' -ForegroundColor Green
} else {
    Write-Host 'The Visitor Group already exists.' -ForegroundColor Green