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 AUTHORITY\authenticated 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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