To get claim for All Authenticated Users in PS you need to use:
$claim = New-SPClaimsPrincipal -EncodedClaim "c:0(.s|true"
for windows users (NT AUTHORITYauthenticated users):
$claim = New-SPClaimsPrincipal -EncodedClaim "c:0!.s|windows"
and for all forms authenticated users (basic asp.net membership provider)
$claim = New-SPClaimsPrincipal -EncodedClaim "c:0!.s|forms:aspnetsqlmembershipprovider"
NOTE: Need to use EnsureUser() or else the add method will not work.
$allAuthenticatedUsers = $web.EnsureUser(“c:0!.s|windows”);
$web.AssociatedVisitorGroup.Users.Add($allAuthenticatedUsers,””,”c:0!.s|windows”,””)
$web.update()
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