To find this just head to any pipeline in pipedrive, then on the right hand side drop down the menu that shows user and filters, select the name of the user that you need the I.d of. Once this has loaded head to the url bar and look at the numeric numbers at the end of the url string, this is your user I.d. Next head to your browser url bar, this should look something like yourcompanyname.pipedrive.com/ in that url string you will notice it says https://yourcompanyname.pipedrive.com/pipeline1/user/####### (# being the user i.d it should be a string of numbers only.