iPhone AND Android phone application required.
The 'App' will use 'Maps' to 'Check-In' a 'Location' with a 'Description'.
That 'Location' needs to somehow be sent to all other phones using the App to update the list of locations from other users.
When another user of the Applicatoin is 'WITHIN 300m' of a 'SHARED LOCATION' the Application must 'BEEP'.
The 'Location' must not last any longer than 90 minutes.
If this explanation is not clear enough please feel free to ask me further questions.