While you cannot customize the default welcome mail for Office 365 Groups, disabling the welcome mail is possible with an extra parameter in the Set-UnifiedGroup cmdlet in Exchange PowerShell: -UnifiedGroupWelcomeMessageEnabled:$false. See https://docs.microsoft.com/en-us/powershell/module/exchange/users-and-groups/set-unifiedgroup
Use this as a part of a custom group creation flow if you would like to restrict and/or customize the welcome mail for many groups across your tenant.Anonymous supported this idea ·Anonymous commented
Since the default information is not valid for us (not using Outlook 2016) and the main part that we use in a Office365 group is the Team site in SharePoint, we need at least the link to the site in the mail.
We are in a multi tenant (20+ domains and all not using the same platform for collaboration, we need to turn of this mail for only some domains. Today we have created a rule under Mail flow to capture this mails so now no information is sent (for our domain).
At least something like when you add a new member to a SharePoint group where you can add info to the mail and also avoid to sent it.