Assumng you mean the WikiTree ID, the answer is a simple and easy process:
When you are on their Profile page, their ID will be the last character string in the URL (address bar) area at the top of your Browser. Also, (and perhaps more importantly when is comes to frequent useage) any place you see a name in green and underlined you will see the WikiTree ID string at the bottom of your browser window when you "mouse over" that name. For instance, when I mouse over your name in the question window as I type this, I can quickly find out that your WikiTree ID is Hesketh-25.
I hope this helps.