Just use this as it accounts for 365 Groups now as well : https://o365reports.com/2019/05/23/export-office-365-distribution-group-members-csv/

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Get Distinguished Name:

get-Mailbox [Username] | fl Name,Distinguishedname

Groups:

get-group -ResultSize Unlimited -Filter ‘Members -eq “[User Distinguished Name]”‘

Distribution Groups:

Get-DistributionGroup -ResultSize Unlimited -Filter ‘Members -eq “[User Distinguished Name]”‘