functions/Get-DbaRegisteredServerGroup.ps1

function Get-DbaRegisteredServerGroup {
    <#
        .SYNOPSIS
            Gets list of Server Groups objects stored in SQL Server Central Management Server (CMS).
 
        .DESCRIPTION
            Returns an array of Server Groups found in the CMS.
 
        .PARAMETER SqlInstance
            SQL Server name or SMO object representing the SQL Server to connect to.
 
        .PARAMETER SqlCredential
            Login to the target instance using alternative credentials. Windows and SQL Authentication supported. Accepts credential objects (Get-Credential)
 
        .PARAMETER Group
            Specifies one or more groups to include from SQL Server Central Management Server.
 
        .PARAMETER ExcludeGroup
            Specifies one or more Central Management Server groups to exclude.
 
        .PARAMETER Id
            Get group by Id(s). This parameter only works if the group has a registered server in it.
 
        .PARAMETER EnableException
            By default, when something goes wrong we try to catch it, interpret it and give you a friendly warning message.
 
            This avoids overwhelming you with "sea of red" exceptions, but is inconvenient because it basically disables advanced scripting.
 
            Using this switch turns this "nice by default" feature off and enables you to catch exceptions with your own try/catch.
 
        .NOTES
            Author: Tony Wilhelm (@tonywsql)
            Tags: RegisteredServer, CMS
 
            Website: https://dbatools.io
            Copyright: (C) Chrissy LeMaire, clemaire@gmail.com
            License: MIT https://opensource.org/licenses/MIT
 
        .LINK
            https://dbatools.io/Get-DbaRegisteredServerGroup
 
        .EXAMPLE
            Get-DbaRegisteredServerGroup -SqlInstance sqlserver2014a
 
            Gets the top level groups from the CMS on sqlserver2014a, using Windows Credentials.
 
        .EXAMPLE
            Get-DbaRegisteredServerGroup -SqlInstance sqlserver2014a -SqlCredential $credential
 
            Gets the top level groups from the CMS on sqlserver2014a, using alternative credentials to authenticate to the server.
 
        .EXAMPLE
            Get-DbaRegisteredServerGroup -SqlInstance sqlserver2014a -Group HR, Accounting
 
            Gets the HR and Accounting groups from the CMS on sqlserver2014a.
 
        .EXAMPLE
            Get-DbaRegisteredServerGroup -SqlInstance sqlserver2014a -Group HR\Development
 
            Returns the sub-group Development of the HR group from the CMS on sqlserver2014a.
    #>

    [CmdletBinding()]
    param (
        [parameter(Mandatory, ValueFromPipeline)]
        [Alias("ServerInstance", "SqlServer")]
        [DbaInstanceParameter[]]$SqlInstance,
        [PSCredential]$SqlCredential,
        [object[]]$Group,
        [object[]]$ExcludeGroup,
        [int[]]$Id,
        [switch]$EnableException
    )
    process {
        foreach ($instance in $SqlInstance) {
            try {
                Write-Message -Level Verbose -Message "Connecting to $instance"
                $server = Get-DbaRegisteredServerStore -SqlInstance $instance -SqlCredential $SqlCredential -EnableException
            }
            catch {
                Stop-Function -Message "Cannot access Central Management Server '$instance'" -ErrorRecord $_ -Continue
            }

            $groups = @()

            if ($group) {
                foreach ($currentgroup in $Group) {
                    Write-Message -Level Verbose -Message "Processing $currentgroup"
                    if ($currentgroup -is [Microsoft.SqlServer.Management.RegisteredServers.ServerGroup]) {
                        $currentgroup = Get-RegServerGroupReverseParse -object $currentgroup
                    }
                    
                    if ($currentgroup -match 'DatabaseEngineServerGroup\\') {
                        $currentgroup = $currentgroup.Replace('DatabaseEngineServerGroup\', '')
                    }
                    
                    if ($currentgroup -match '\\') {
                        $split = $currentgroup.Split('\\')
                        $i = 0
                        $groupobject = $server.DatabaseEngineServerGroup
                        do {
                            if ($groupobject) {
                                $groupobject = $groupobject.ServerGroups[$split[$i]]
                                Write-Message -Level Verbose -Message "Parsed $($groupobject.Name)"
                            }
                        }
                        until ($i++ -eq $split.GetUpperBound(0))
                        if ($groupobject) {
                            $groups += $groupobject
                        }
                    }
                    else {
                        try {
                            $thisgroup = $server.DatabaseEngineServerGroup.ServerGroups[$currentgroup]
                            if ($thisgroup) {
                                Write-Message -Level Verbose -Message "Added $($thisgroup.Name)"
                                $groups += $thisgroup
                            }
                        }
                        catch { }
                    }
                }
            }
            else {
                Write-Message -Level Verbose -Message "Added all root server groups"
                $groups = $server.DatabaseEngineServerGroup.ServerGroups
            }

            if ($Group -eq 'DatabaseEngineServerGroup') {
                Write-Message -Level Verbose -Message "Added root group"
                $groups = $server.DatabaseEngineServerGroup
            }

            if ($ExcludeGroup) {
                $excluded = Get-DbaRegisteredServer $server -Group $ExcludeGroup
                Write-Message -Level Verbose -Message "Excluding $ExcludeGroup"
                $groups = $groups | Where-Object { $_.Urn.Value -notin $excluded.Urn.Value }
            }

            if ($Id) {
                Write-Message -Level Verbose -Message "Filtering for id $Id. Id 1 = default."
                if ($Id -eq 1) {
                    $groups = $server.DatabaseEngineServerGroup | Where-Object Id -in $Id
                }
                else {
                    $groups = $server.DatabaseEngineServerGroup.GetDescendantRegisteredServers().Parent | Where-Object Id -in $Id
                }
            }
            $server.ServerConnection.Disconnect()
            foreach ($groupobject in $groups) {
                Add-Member -Force -InputObject $groupobject -MemberType NoteProperty -Name ComputerName -value $server.ComputerName
                Add-Member -Force -InputObject $groupobject -MemberType NoteProperty -Name InstanceName -value $server.InstanceName
                Add-Member -Force -InputObject $groupobject -MemberType NoteProperty -Name SqlInstance -value $server.SqlInstance

                Select-DefaultView -InputObject $groupobject -Property ComputerName, InstanceName, SqlInstance, Name, DisplayName, Description, ServerGroups, RegisteredServers
            }
        }
    }
}