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Introduction:

# German:
Get-LocalGroupMember -Group "Administratoren"

# English
# Get-LocalGroupMember -Group "Administrators"

# Add a local user to the local administrator group
Add-LocalGroupMember -Group "Administratoren" -Member "TOMHOMEDESK\LokalAdmin"

# Check the result:
Get-LocalGroupMember -Group "Administratoren"

Conclusion:

Sources:

https://www.isunshare.com/windows-server/add-a-user-to-local-administrator-group.html

Powershell Script to Add a User to a Local Admin Group
In this blog post, I cover adding user accounts and groups to the local administrator group using Powershell.