When you create a Microsoft 365 Group in Teams it will be hidden from Outlook by default. If you attempt to unhide it via the Microsoft 365 Admin Portal that won’t work either. This can only be fixed/changed using PowerShell.

#### POWERSHELL COMMANDS ####

## Ensure all downloads are signed by a trusted publisher ##
Set-ExecutionPolicy RemoteSigned

## Install the Exchange Online Management Module ##
Install-Module -Name ExchangeOnlineManagement

## Import the Exchange Online Module ##
Import-Module ExchangeOnlineManagement

## Connect to the Exchange Tenant ##
Connect-ExchangeOnline -UserPrincipalName {username} -ShowProgress $true

## Enable one archive mailbox ##
Set-UnifiedGroup -Identity "{group name}" -HiddenFromExchangeClientsEnabled:$false

## Disconnect Session ##
Disconnect-ExchangeOnline