internal/functions/Resolve-UserString.ps1

function Resolve-UserString {
    <#
    .SYNOPSIS
        Converts usernames or email addresses into the user targeting segment of the Rest Api call url.
 
    .DESCRIPTION
        Converts usernames or email addresses into the user targeting segment of the Rest Api call url.
 
    .PARAMETER User
        The user to convert
 
    .EXAMPLE
        PS C:\> Resolve-UserString -User $User
 
        Resolves $User into a legitimate user targeting string element.
    #>

    [OutputType([System.String])]
    [CmdletBinding()]
    param (
        [string]
        $User
    )

    if ($User -eq 'me' -or (-not $User)) { return 'me' }
    elseif ($User -like "users/*") { return $User }
    else { return "users/$($User)" }
}