Login to Office 365
https://admin.microsoft.com/
Go to Teams & Groups > Active Teams & Groups
Search for the email
Click on the email you want to change
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Click Edit under Members
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Click Add Members
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Search for User
Check box for their user
Click Add
--Sometimes the name provided isn’t exact and you can find the correct one in 365 or AD or you can ask for the users email address.
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Confirm user is listed.
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Login to AD.
Right click the domain and select find.
Search for the related group.
--There will be two. One for SendAs and one for FullAccess
Add the user.