I have developed an android "Group Chat" app. Me and 20 of my friends are using this app to group chat with each other. When I look at the firebase database, I obviously see 21 users (including myself). However, I can't know whether all of them actually have the app installed on their phones, or some of them have uninstalled it. Is there any way I can differentiate among my uses? Generally speaking, can I "flag" my users as, say, *active*, *inactive*, and the like?