$web = get-spweb "https://yourWeb"
$SourceGroup = $web.groups | where {$_.name -eq "XY Owners" }
$TargetGroup = $web.groups | where {$_.name -eq "XY Members" }
foreach ($user in $Sourcegroup.users)
{
Write-Host "Moving $user from $SourceGroup to $TargetGroup"
$TargetGroup.AddUser($user)
#$SourceGroup.RemoveUser($user)
}