The way to do this, when you need to add AND delete users atomically from your waiting list, is to use Firebase as intended, and create a node (tag) for each user that is added. This way firebase will be able to handle (for you) any simultaneous button presses. You would use the Firebase REST api for this, and a web component in your app. You use POSTText to generate a unique key for each user.
FIREBASE DATA
BLOCKS
SCREEN
an example firebase url (as found in the obfuscated blocks above):
