Is it possible to create a single task that notifies multiple users (3 - 4) to prompt them so one of them then actions it? I don't want to create multiple tasks, just one task for multiple users. So all the users are notified and have the task in their pop-up reminders but as soon as one of them actions it, it drops off for everyone & not just the user who actioned it. Any ideas how this can be done?
You'll have to do this through code I'm afraid. Each task is assigned to exactly one user, so multiple users means multiple tasks. The way I've handled this in the past is to have a trigger on the task sobject, and when a task is marked as complete, find all other tasks matching the criteria (same name, for example) and remove them. I used the remove as I only wanted the history of who did the work, not who could potentially do it. There is an element of fragility about this, as users can change their task instance and stop it matching the criteria from the closed task.
There's an idea to support the functionality you are looking for at :