Welcome message to the Office 365 (Unified) group member is by default enabled, but some time user may wish not to inform the recipient about their addition in the group. as it could be annoying for the recipient in case of performing addition to multiple groups.
To disable a welcome message for Unified group in office 365 run the following command.
POWERSHELL
Set-UnifiedGroup TestGroup -UnifiedGroupWelcomeMessageEnabled:$false