This is a little tricky. I was looking for a way to list all the users that you see in Site Settings –> People and Groups, but I found more info than I needed. This post will be updated once I can track down the actual site users.

The script uses the Users and Groups web service to pull all the users in the site collection.
UserGroup.asmx

In my case, the LoginName is setup like this: domain\username

First try:

$webServ = "http://sharepointed/sites/MySiteCollection/_vti_bin/UserGroup.asmx"
$Service = New-WebServiceProxy -UseDefaultCredential -uri $webServ 
$Users = $Service.GetUserCollectionFromSite().Users 
$UserNames = New-Object System.Collections.Generic.List[System.Object]

$Users.User | ForEach-Object {
	$spUser = $_.LoginName.Split('\')[1]
	$UserNames.Add($spUser)
}

Found the answer to my question. Using the Lists.asmx service, I was able to query the UserInfo list for all the site users.

$webServ = "http://sharepointed/sites/MySiteCollection/_vti_bin/Lists.asmx"
$Service = New-WebServiceProxy -UseDefaultCredential -uri $webServ 

$UserNames = New-Object System.Collections.Generic.List[System.Object]

$listname = 'UserInfo'
$listItems = $Service.GetListItems($listname, $null, $null, $null, $null, $null, $null)

for ($counter = 0;$counter -lt $listItems.data.row.Count;$counter++)
{
	$UserNames += $listItems.data.row[$counter].ows_Name
}

Using c# to get at the information. Here I output the LoginName (domain\userName).

            #create a Web Reference to http://yoursiteURL/_vti_bin/usergroup.asmx?wsdl
            #in my case, i named the reference wsUsersGroups
            wsUsersGroups.UserGroup _WSUsersGroups = new wsUsersGroups.UserGroup();
            _WSUsersGroups.Url = "http://sharepointSite/sites/SiteCollectionName/_vti_bin/usergroup.asmx";
            _WSUsersGroups.Credentials = System.Net.CredentialCache.DefaultCredentials;
            XmlNode ndUsers = _WSUsersGroups.GetAllUserCollectionFromWeb();

            StringReader rdrGroups = new StringReader(ndUsers.OuterXml);
            DataSet dsGroups = new DataSet();
            dsGroups.ReadXml(rdrGroups);

            StringBuilder sb = new StringBuilder();

            foreach (DataRow item in dsGroups.Tables[1].Rows)
            {
                sb.AppendLine(item[3].ToString());
            }

            File.WriteAllText("C:\\Users\\myname\\Desktop\\siteUSers.csv", sb.ToString());