Where can I find mine or another player's ID number?

Every player has a unique ID number. You can find your own ID number by going into your profile and looking under your name. It is important if you are reporting and bugs to always quote this or we cannot check your account. 

You can do the same way in which you found your own ID, by checking in their profile or check in the URL bar when in their profile and it will show at the end of it.