If you want it to pop up every time someone joins then make a GUI and then make a proecedure with the global trigger: "Player joins the world" and put the procedure block "Open GUI" under it and it will open everytime that player joins the world.
If you only want it to pop up once you can make a boolean variable like "openedGUI" which is set to false by default and then insert an if statement in the procedure I mentioned above which checks if it is false and then opens the menu and sets it to true meaning it cannot be opened again.
If you want it to pop up every time someone joins then make a GUI and then make a proecedure with the global trigger: "Player joins the world" and put the procedure block "Open GUI" under it and it will open everytime that player joins the world.
If you only want it to pop up once you can make a boolean variable like "openedGUI" which is set to false by default and then insert an if statement in the procedure I mentioned above which checks if it is false and then opens the menu and sets it to true meaning it cannot be opened again.