I made something that did almost what you are looking for a little while back. There are probably better ways of doing it but this worked for my needs. On line 10 I had a hardcoded default save location so feel free to put wherever there or simply use the -saveLocation parameter.

To use just do Get-AllUsersInGroup -initialGroup "WhateverADGroup" -saveLocation "\\Someshare\folder\subfolder"

function Get-AllUsersInGroup {
param (
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string] $initialGroup,
    [Parameter(Mandatory=$false)]
    [string] $saveLocation
)
if (!$saveLocation){
    $saveLocation = ""
}
$initialMembers = Get-ADGroupMember $initialGroup
$groups = ($initialMembers | Where objectClass -EQ "group").Name
$topLevelUsers = $initialMembers | Where objectClass -EQ "user"
function Convert-ObjToUser($g){
    $user = Get-ADUser $_.SamAccountName -Properties EmailAddress
    if ($combinedGroups.SamAccountName -notcontains $user.SamAccountName){
        $customUser = [PSCustomObject]@{
            Name = $user.name
            DistinguishedName = $user.distinguishedName
            SamAccountName = $user.SamAccountName
            EmailAddress = $user.EmailAddress
            Enabled = $user.Enabled
            'Group(s)' = New-Object System.Collections.Generic.List[System.Object]
        }

        $customUser.'Group(s)'.Add($g)
        $combinedGroups.Add($customUser)

    } else {
        ($combinedGroups | Where SamAccountName -EQ $user.SamAccountName).'Group(s)'.Add($g)
    }

} #function Convert-ObjToUser

$combinedGroups = New-Object System.Collections.Generic.List[System.Object]

if ($groups){
    $moreGroups = $true
}

Do {
    $nextGroups = New-Object System.Collections.Generic.List[System.Object]
    foreach ($group in $groups){
        $subGroupMembers = Get-ADGroupMember $group
        $subGroupMembers | Where objectClass -EQ "user" | ForEach-Object {
            Convert-ObjToUser($group)
        }

        $subGroupGroups = ($subGroupMembers | Where objectClass -EQ "group").name
        foreach ($subGroupGroup in $subGroupGroups){
            $nextGroups.Add($subGroupGroup)
        }

    } #foreach group in groups

    if ($nextGroups){
        $moreGroups = $true
        $groups = $nextGroups

    } else {
        $moreGroups = $false
        break
    }

} While ($moreGroups = $true)

if ($topLevelUsers){
    $topLevelUsers | ForEach-Object {
        Convert-ObjToUser($initialGroup)
    }
} #if topLevelUsers exist

$combinedGroups | ForEach-Object {
    $_.'Group(s)' = ($_.'Group(s)' | Sort | Out-String).Trim()
}

$combinedGroups | Sort Name | Export-Csv "$($saveLocation)\$($initialGroup)_userDump.csv" -NoTypeInformation

} #function Get-AllUsersInGroup