I didn't fully understand you goal, but did you consider a single timer with a resolution low enough to be used for all of the "users"?
I mean, if your timer ticks once per minute, in its sub I can verify which users need to be called based on counters specific for each user (e.g. user A once every 5 ticks, meaning once every 5 minutes, user B once evry 12 ticks and so on).