Agree with the suggestions above from AshokM, we can use the Update-DistributionGroupMember to meet this requirement
Here is also a related thread discussed about this similar question as yours: Add multiple members to distribution group with one call?
$NewMembers = Get-Content <path to member list text file>
$CurrentMembers = (Get-DistributionGroupMember -Identity GroupName).PrimarySmtpAddress | where {$_ -ne ""}
$NewMemberList = $CurrentMembers + $NewMembers
Update-DistributionGroupMember -Identity GroupName -Members $NewMemberList -Confirm:$false
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