This function will return all domain users in the specified group, including recursive memberships. This also includes users in special groups such as the "Domain Users" group. Users are part of special groups via the SID instead of the members attribute. Therefore `Get-ADGroupMembers -Recursive` doesn't return users in these special groups.

Get-AllADGroupUsersRecursively.ps1

Function Get-AllADGroupUsersRecursively {
<#
.SYNOPSIS
    This script will identify all AD users in a specific group recursively (including special groups such as "Domain Users").

.DESCRIPTION
    This script will identify all AD users in a specific group recursively (including special groups such as "Domain Users").
 
.NOTES
    Name: Get-AllADGroupUsersRecursively
    Author: Tijme Gommers
    Version: 1.0
    DateCreated: 05/31/2021

.PARAMETER Group
    The group name to start searching in

.EXAMPLE
    Get-AllADGroupUsersRecursively HR_Department_Network_Share

.LINK
    https://twitter.com/tijme
#>

    [CmdletBinding()]
    Param(
        [Parameter(
            Mandatory = $True, 
            HelpMessage = "Specify the group name to start searching in",
            ValueFromPipeline = $True,
            ValueFromPipelineByPropertyName = $True,
            Position = 0
        )] [ValidateNotNullOrEmpty()] [string] $Group
    )
 
    Begin {
        $groupsHT = @{} # Parsed groups hashtable
        $usersHT = @{} # Parsed users hashtable

        if ($Group -is [String]) {
            $Group = Get-ADGroup $Group
        }
    }

    Process {
        if($groupsHT.Contains($Group) -eq $true) {
            return
        }

        $groupsHT.Add($Group, 1)
        $members = Get-ADGroupMember $Group

        foreach($member in $members) {
            if ($member.objectClass -eq "group") {
                Get-AllADGroupMemberRecursively $member
            } else {
                $usersHT.Add($member, 1)
            }
        }
    }

    End {
        return $usersHT.Keys
    }

}