Get-DistributionGroup -Identity "SourceGroup" | Copy-DistributionGroupMembers -DestinationGroup "DestinationGroup"

Get-DistributionGroup -Identity "SourceGroup" | Copy-DistributionGroupMembers -DestinationGroup "DestinationGroup" -RemoveExtraMembers

<#
.SYNOPSIS
    Copies missing members from one Distribution Group to another and optionally removes extra members.

.DESCRIPTION
    This function retrieves all members of the source Distribution Group and compares them to the members 
    of the destination Distribution Group. Any members that are not already in the destination group are added.
    The function accepts the source group identity via pipeline input and takes the destination group as a parameter.

.PARAMETER SourceGroup
    The identity of the source Distribution Group. Accepts pipeline input.

.PARAMETER DestinationGroup
    The identity of the destination Distribution Group.

.PARAMETER RemoveExtraMembers
    Switch to remove members from the destination group that are not in the source group.

.EXAMPLE
    Get-DistributionGroup -Identity "SourceGroup" | Copy-DistributionGroupMembers -DestinationGroup "DestinationGroup"

    This example retrieves the group "SourceGroup" and pipes its identity into the function to add any missing 
    members to "DestinationGroup".

.EXAMPLE
    Get-DistributionGroup -Identity "SourceGroup" | Copy-DistributionGroupMembers -DestinationGroup "DestinationGroup" -RemoveExtraMembers

    This example retrieves the group "SourceGroup" and pipes its identity into the function to add any missing 
    members to "DestinationGroup" and remove any extra members from "DestinationGroup".

.NOTES
    For Active Directory groups, replace Get-DistributionGroupMember / Add-DistributionGroupMember with 
    Get-ADGroupMember / Add-ADGroupMember accordingly.
#>

function Copy-DistributionGroupMembers {
    [CmdletBinding(SupportsShouldProcess)]
    param(
        # Accepts pipeline input for the source group identity (e.g. group name, alias or distinguished name)
        [Parameter(Mandatory = $true, ValueFromPipeline = $true, Position = 0)]
        [string]$SourceGroup,
        
        # Destination group identity (e.g. group name, alias or distinguished name)
        [Parameter(Mandatory = $true, Position = 1)]
        [string]$DestinationGroup,

        # Switch to remove members from the destination group that are not in the source group
        [Parameter(Mandatory = $false)]
        [switch]$RemoveExtraMembers
    )

    begin {
        $results = @()
        # Retrieve the current members of the destination group for a quick lookup.
        try {
            $destinationMembers = Get-DistributionGroupMember -Identity $DestinationGroup -ErrorAction Stop
            $destinationEmails = $destinationMembers | ForEach-Object {
                $_.PrimarySmtpAddress.ToString().ToLower()
            }
        }
        catch {
            Write-Error "Failed to retrieve members of destination group '$DestinationGroup'. Error: $_"
            return
        }
    }

    process {
        # Retrieve members of the source group
        try {
            $sourceMembers = Get-DistributionGroupMember -Identity $SourceGroup -ErrorAction Stop
            $sourceEmails = $sourceMembers | ForEach-Object {
                $_.PrimarySmtpAddress.ToString().ToLower()
            }
        }
        catch {
            Write-Error "Failed to retrieve members of source group '$SourceGroup'. Error: $_"
            return
        }

        foreach ($member in $sourceMembers) {
            $memberEmail = $member.PrimarySmtpAddress.ToString().ToLower()

            if (-not $memberEmail) {
                Write-Verbose "Skipping member '$($member.Name)' as no PrimarySmtpAddress was found."
                continue
            }

            # Add the member if not already present in the destination group
            if (-not ($destinationEmails -contains $memberEmail)) {
                if ($PSCmdlet.ShouldProcess("$DestinationGroup", "Add $($member.Name) ($memberEmail)")) {
                    try {
                        Add-DistributionGroupMember -Identity $DestinationGroup -Member $member.Identity -ErrorAction Stop
                        Write-Verbose "Added '$($member.Name)' to '$DestinationGroup'."
                        $results += [PSCustomObject]@{
                            Action = "Added"
                            Member = $member.Name
                            Email  = $memberEmail
                        }
                    }
                    catch {
                        Write-Warning "Failed to add '$($member.Name)' to '$DestinationGroup'. Error: $_"
                    }
                }
            }
            else {
                Write-Verbose "'$($member.Name)' is already a member of '$DestinationGroup'."
            }
        }

        if ($RemoveExtraMembers) {
            foreach ($member in $destinationMembers) {
                $memberEmail = $member.PrimarySmtpAddress.ToString().ToLower()

                if (-not ($sourceEmails -contains $memberEmail)) {
                    if ($PSCmdlet.ShouldProcess("$DestinationGroup", "Remove $($member.Name) ($memberEmail)")) {
                        try {
                            Remove-DistributionGroupMember -Identity $DestinationGroup -Member $member.Identity -ErrorAction Stop
                            Write-Verbose "Removed '$($member.Name)' from '$DestinationGroup'."
                            $results += [PSCustomObject]@{
                                Action = "Removed"
                                Member = $member.Name
                                Email  = $memberEmail
                            }
                        }
                        catch {
                            Write-Warning "Failed to remove '$($member.Name)' from '$DestinationGroup'. Error: $_"
                        }
                    }
                }
            }
        }
    }

    end {
        $results
    }
}