PLEASE READ ALL THE INSTRUCTIONS! DISCLAIMER: No, this does not use the Scratch API to detect whether a user is online or not, so for example, it can't detect whether Griffpatch is online or anything like that. The user has to have a tab open with the project running in order to do this. Click the flag and you can tell if I'm online or offline! This will only work if I have the tab open though. :P You will have to remix to test for others because clicking see inside disables cloud variables. But be warned, it only works if they have your remix running. So basically, the user that you test for has to be running your remix in a tab.
Credit to EatNYeet for the original project