How to remove the Welcome Message when a new member joins a group.

Here's a quick PowerShell script I threw together that may help:

$UserCredential = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection

Import-PSSession $Session

#Get all Office 365 Groups that have have the Welcome Message enabled
$O365Groups = Get-UnifiedGroup | Where-Object{$_.WelcomeMessageEnabled -eq $true}

#Iterate through the Groups, disabling the Welcome Message
foreach ($group in $O365Groups)
    {
    Write-Host "Disabling Welcome Message on O365 Group: " -NoNewline; Write-Host $group.DisplayName -ForegroundColor Cyan
    Set-UnifiedGroup $group.Identity -UnifiedGroupWelcomeMessageEnabled:$false
    }

#Close the Session
Remove-PSSession $Session

I've commented it, but basically it gathers the Office 365 Groups in your Tenant that have the Welcome Message enabled and then loops through them disabling the Welcome Message. You can run this as needed.

Hope it's useful for someone out there. :)