Hi, @SR VSP ,
Just like Andy said, for now MS does not have the plan to have this feature to customize the welcome Email. The suggested approach will be disabling this welcome email and add a custom flow to send an Email.
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://learn.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.
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