Hi EricPLD,
Thanks for posting here, I'm very willing to help you.
You can only use PowerShell cdml in order to disable your welcome message notification to the user. Please follow the instruction below:
- Connect to your Office 365 Admin using PowerShell(about how to connect please refer to this article:https://docs.microsoft.com/en-us/powershell/exchange/connect-to-exchange-online-powershell?view=exchange-ps)
- Execute the following command Get-UnifiedGroup <group name> | fl WelcomeMessageEnabled to see if its set to True
- Set it to be False in order to turn off notifications to the newly added users
- Get-UnifiedGroup <group name> | Set-UnifiedGroup -UnifiedGroupWelcomeMessageEnabled:$false
Hope this can help you if you need any further help please feel free to let me know and have a great weekends.
Best Regards,
Regina