Powershell Script to Add Multiple Users in SharePoint Owners group

I think, best way is create a group and add users in it then make that group as owner of your target group.

To assign group as a onwer of another group.

$web = Get-SPWeb http://webUrl
$groupOwner = $web.SiteGroups["GroupA"] #new group,this group will own "GroupB", 
$groupTarget = $web.SiteGroups["GroupB"]
$groupTarget.Owner = $groupOwner
$groupTarget.Update()

Change SharePoint group owner with Powershell to a SharePoint group

To create group and add user in it, this code will read from an XML file and create the group along with users.

XML file format

?xml version="1.0"?>
<Groups>
    <Group name="Approvers" description="">
        <Users>
            <User>DOMAIN\pchilds</User>
            <User>DOMAIN\tsmith</User>
        </Users>
    </Group>
</Groups>

Here is powershell to create the groups

#Get Site and Web objects
$site = Get-SPSite “http://portal”
$web = $site.RootWeb

#Get XML file containing groups and associated users
$groupsXML = [xml] (Get-Content ("C:\Path\Groups.XML"))

#Walk through each group node defined in the XML file
$groupsXML.Groups.Group | ForEach-Object {
    #Check to see if SharePoint group already exists in the site collection
    if ($web.SiteGroups[$_.name] -eq $null)
    {
        #If the SharePoint group doesn't exist already - create it from the name and description values at the node
        $newGroup = $web.SiteGroups.Add($_.name, $web.CurrentUser, $null, $_.description)
    }

    #Get SharePoint group from the site collection
    $group = $web.SiteGroups[$_.name]

    #Add the users defined in the XML to the SharePoint group
    $_.Users.User | ForEach-Object {
        $group.Users.Add($_, "", "", "")
    }
}

#Dispose of Web and Site objects
$web.Dispose()
$site.Dispose()

Ref: http://get-spscripts.com/2010/07/creating-multiple-sharepoint-groups-and.html

Assuming you have user in txt file...below can be used...

    $loc = Get-Location     
    $Users = Get-Content "$loc\Data\Users.txt"  
    $siteCollUrl = "http://sharepoint/sites/sitecollection/subsite"
    $groupname ="Owner Group" // valid sharepoint group name
    $web = Get-SPWeb -identity $siteCollUrl
    $group = $SPWeb.SiteGroups[$groupname]
    foreach ($User in $Users) { 
        $web.EnsureUser($User)  
        Set-SPUser -Identity $User -Web $siteCollUrl -Group $group  
    }