i've seen a few people post that they've had no guardians in there session weather it's at the tower or in one of the worlds, i've had this problem myself and it seems to be the xb1 players affected, now this is actually not a bungie/destiny bug it's to do with the standby feature on your xb1, when you turn off your xbox if you haven't actually performed the step of "quitting" destiny via the dashboard and you just turn it off, when you turn it back on it's been in stand-by and reboots your previous settings/servers/game progress thus not allowing you to enter the updated playlist/server that everyone else is currently playing on.
to fix this, simply go to the dashboard, select options on destiny then select quit. relaunch the game from new and you'll be added to the updated playlist and all the other players will magically re-appear!
hope this helps anyone who's had the same problem and thought that they were the final remaining survivor of the darkness!
Happy grinding people ;)