You cannot send all the requests at the same point of time unless you put every request in a new tab.
But there is a much better alternative for this. In the Test Script, you can create an array and store all your userIDs in that array. Run a loop, make use of “setNextRequest()” method to point the same request every time you run the loop until all your userIDs are done. This helps you to send the requests approximately at the same time.