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Dear David,
Good day!!
As per the description shared, I would like to confirm whether you want to prevent to send the emails to the users that are added into the SharePoint Groups in the SharePoint Online site.
If yes, there is no any direct setting or default setting in the SharePoint Online to uncheck the option called “Send an email invitation” when either Granting Access or adding the users to the SharePoint groups. However, when you are adding, you can uncheck the option to send an email invitation:
However, if you are adding the users from the Site Permissions in the SharePoint Online site via Settings> Site Permissions> Add members, there is no option to prevent from the email invitation for the added users.
If there is any misunderstanding, I request you to share the screenshots where you are trying to add the users and prompts/windows you encounter when following the process for adding the users.
Thanks for your patience and understanding. Have a good day!!
Best Regards,
Sophia