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.Ken N commented
+1 here, We use these for our project initiations, be nice to add some customizatins on this. At least the basic message. "Project X has been Initiated"